An 18-month-old girl is brought to the pediatrician’s office for failure to thrive and developmental delay. The patient’s mother says she has not started speaking and is just now starting to pull herself up to standing position. Furthermore, her movement appears to be restricted. Physical examination reveals coarse facial features and restricted joint mobility. Laboratory studies show increased plasma levels of several enzymes. Which of the following is the underlying biochemical defect in this patient?
Q2
An 18-month-old boy of Ashkenazi-Jewish descent presents with loss of developmental milestones. On ocular exam, a cherry-red macular spot is observed. No hepatomegaly is observed on physical exam. Microscopic exam shows lysosomes with onion-skin appearance.
What is the most likely underlying biochemical abnormality?
Q3
A 4-month-old male infant is brought in because he rejects food and is losing weight. He had several upper respiratory tract infections during the last 2 months. Upon examination, hepatosplenomegaly is noted, as well as mild hypotonia. During the next few weeks, hepatosplenomegaly progresses, the boy fails to thrive, and he continues to reject food. He has a blood pressure of 100/70 mm Hg and heart rate of 84/min. Blood tests show pancytopenia and elevated levels of transaminases. Slit lamp examination shows bilateral cherry-red spots on the macula. Chest X-ray shows a reticulonodular pattern and calcified nodules. Biopsy of the liver shows foamy histiocytes. What is the most likely diagnosis?
Q4
A 10-month-old boy is brought to the physician by his mother for evaluation of abnormal growth and skin abnormalities. His mother has also noticed that his eyes do not fully close when sleeping. He is at the 24th percentile for height, 17th percentile for weight, and 29th percentile for head circumference. Physical examination shows wrinkled skin, prominent veins on the scalp and extremities, and circumoral cyanosis. Genetic testing shows a point mutation in a gene that encodes for a scaffold protein of the inner nuclear membrane. The mutation causes a deformed and unstable nuclear membrane, which leads to premature aging. Which of the following is most likely to be the defective protein?
Q5
A 7-month-old boy is brought to the pediatrician by his parents due to progressively worsening weakness for the last three months. The parents also describe the boy as having an exaggerated response when startled as well as diminishing response to visual stimuli. At birth, the boy was healthy and remained as such for the first few months of life. The mother says pregnancy was unremarkable, and the boy was born at 39 weeks with no complications during delivery. He is up to date on his vaccinations. The boy's grandparents immigrated from an eastern European country. Physical examination reveals hyperreflexia. Abdominal examination reveals no abnormalities. On fundoscopy, the following is seen. Which of the following is most likely deficient in this patient?
Q6
A 1-year-old boy is brought to his pediatrician for a follow-up appointment. He was recently diagnosed with failure to thrive and developmental delay. His weight is 7 kg (15.4 lb), height is 61 cm (24 in), and head circumference is 42 cm (16.5 in). The patient’s father had a younger sister who suffered from mental and physical delay and died at a very young age. The patient was able to raise his head at the age of 7 months and began to sit alone only recently. He babbles, coos, and smiles to other people. On presentation, his blood pressure is 75/40 mm Hg, heart rate is 147/min, respiratory rate is 28/min, and temperature is 36.4°C (97.5°F). He has a coarse face with small deep orbits, proptotic eyes, big lips, and gingival hyperplasia. His skin is pale with decreased elasticity. His lung and heart sounds are normal. Abdominal examination reveals diminished anterior abdominal wall muscle tone and hepatomegaly. Muscle tone is increased in all groups of muscles on both upper and lower extremities. The physician becomes concerned and performs testing for the suspected hereditary disease. A blood test shows increased lysosomal enzyme concentration in the serum and decreased N-acetylglucosamine-1-phosphotransferase (GlcNAc phosphotransferase) activity within the leukocytes. Which of the statements listed below describes the mechanism of the patient’s condition?
Q7
A scientist wants to determine if a specific fragment is contained within genome X. She uses a restriction enzyme to digest the genome into smaller fragments to run on an agarose gel, with the goal of separating the resulting fragments. A nitrocellulose blotting paper is then used to transfer the fragments from the agarose gel. A radiolabeled probe containing a complementary sequence to the fragment she is searching for is incubated with the blotting paper. Which of the following is the RNA equivalent of this technique?
Q8
A 70-year-old man comes to the physician because of progressive fatigue and lower back pain for the past 4 months. The back pain worsened significantly after he had a minor fall while doing yard work the previous day. For the past year, he has had a feeling of incomplete emptying of his bladder after voiding. His vital signs are within normal limits. Examination shows bilateral paravertebral muscle spasm, severe tenderness over the second lumbar vertebra, and mild tenderness over the lower thoracic vertebrae. Neurologic examination shows no abnormalities. His hemoglobin is 10.5 g/dl, alkaline phosphatase is 110 U/L, and serum calcium is 11.1 mg/dl. An x-ray of the skull is shown. Which of the following is the most appropriate next step in diagnosis?
Q9
An 8-month-old female infant from a first-degree consanguineous couple was brought to the physician because the mother noticed abnormalities in the growth of her child as well as the different lengths of her child's legs. The infant had gingival hyperplasia, restricted movement in both shoulders, a prominent, pointed forehead, and enophthalmos with a slight opacity in both corneas. A blood test revealed 10 fold higher than normal levels of the following enzymes: N-acetyl-ß-glucosaminidase, ß-glucuronidase, ß-hexosaminidase A, and alkaline phosphatase. Which of the following is most likely deficient in this patient?
Q10
A 47-year-old homeless man is brought to the emergency department by police, who found him sleeping by the side of the street. He is somnolent and confused and is unable to give a reliable history. His medical history is unobtainable. Vital signs include: temperature 36.9°C (98.4°F), blood pressure 112/75 mm Hg, and pulse 85/min. Physical examination reveals that he has severe truncal ataxia and horizontal gaze palsy with impaired vestibulo-ocular reflexes. Muscle stretch reflexes and motor strength are normal. He has no sensory deficits. Which of the following best represents the most likely etiology of this patient’s condition?
Lysosomal membrane protein disorders US Medical PG Practice Questions and MCQs
Question 1: An 18-month-old girl is brought to the pediatrician’s office for failure to thrive and developmental delay. The patient’s mother says she has not started speaking and is just now starting to pull herself up to standing position. Furthermore, her movement appears to be restricted. Physical examination reveals coarse facial features and restricted joint mobility. Laboratory studies show increased plasma levels of several enzymes. Which of the following is the underlying biochemical defect in this patient?
A. Congenital lack of lysosomal formation
B. Inappropriate protein targeting to endoplasmic reticulum
C. Failure of mannose phosphorylation (Correct Answer)
D. Inappropriate degradation of lysosomal enzymes
E. Misfolding of nuclear proteins
Explanation: ***Failure of mannose phosphorylation***
- The constellation of **failure to thrive**, **developmental delay**, **coarse facial features**, restricted joint mobility, and elevated plasma enzymes in an 18-month-old girl is highly suggestive of **I-cell disease** (mucolipidosis type II).
- **I-cell disease** is caused by the deficiency of **N-acetylglucosaminyl-1-phosphotransferase**, an enzyme responsible for phosphorylating mannose residues on lysosomal enzymes, which is crucial for proper targeting to the lysosome.
*Congenital lack of lysosomal formation*
- **Lysosomes** are present in this condition, but their enzymes are misdirected.
- A congenital lack of lysosomal formation would present with even more severe and widespread cellular dysfunction, possibly incompatible with life beyond early embryonic stages.
*Inappropriate protein targeting to endoplasmic reticulum*
- Proteins destined for the endoplasmic reticulum (ER) are typically targeted by an N-terminal signal peptide and then processed within the ER.
- While ER dysfunction can cause various disorders, the specific symptoms and enzyme elevations point away from a primary ER targeting defect related to lysosomal enzymes.
*Inappropriate degradation of lysosomal enzymes*
- In I-cell disease, lysosomal enzymes are synthesized but are **not properly targeted to the lysosomes**; instead, they are secreted into the bloodstream, leading to their elevated plasma levels.
- While some degradation might occur, the primary issue is mis-packaging and secretion, not increased degradation within the cell.
*Misfolding of nuclear proteins*
- Misfolding of nuclear proteins can lead to a variety of genetic disorders and cellular stress responses, but the clinical presentation, particularly the accumulation of undegraded material and elevated plasma lysosomal enzymes, is not characteristic of primary nuclear protein misfolding.
- The pathology in I-cell disease centers on lysosomal dysfunction rather than nuclear protein abnormalities.
Question 2: An 18-month-old boy of Ashkenazi-Jewish descent presents with loss of developmental milestones. On ocular exam, a cherry-red macular spot is observed. No hepatomegaly is observed on physical exam. Microscopic exam shows lysosomes with onion-skin appearance.
What is the most likely underlying biochemical abnormality?
A. Accumulation of ceramide trihexoside
B. Accumulation of glucocerebroside
C. Accumulation of galactocerebroside
D. Accumulation of sphingomyelin
E. Accumulation of GM2 ganglioside (Correct Answer)
Explanation: ***Accumulation of GM2 ganglioside***
- This constellation of symptoms—**loss of developmental milestones**, **cherry-red macular spot**, absence of hepatomegaly, and **lysosomes with onion-skin appearance** in an individual of **Ashkenazi-Jewish descent**—is classic for **Tay-Sachs disease**.
- **Tay-Sachs disease** is caused by a deficiency of **hexosaminidase A**, leading to the accumulation of **GM2 ganglioside** in neuronal lysosomes.
*Accumulation of ceramide trihexoside*
- This refers to **Fabry disease**, which is an **X-linked disorder** presenting in adolescence or adulthood with acroparesthesias, angiokeratomas, and renal/cardiac complications.
- While it involves a lysosomal storage, its clinical presentation and the absence of a cherry-red spot differentiate it from the case described.
*Accumulation of glucocerebroside*
- This is characteristic of **Gaucher disease**, which is caused by a deficiency in **glucocerebrosidase**.
- Key features include **hepatosplenomegaly**, bone pain, and pancytopenia, which are not consistent with the patient's presentation.
*Accumulation of galactocerebroside*
- This describes **Krabbe disease**, a **globoid cell leukodystrophy** caused by a deficiency in galactocerebrosidase.
- Krabbe disease primarily affects the **myelin sheath** in the nervous system, leading to neurological degeneration but typically does not present with a cherry-red macular spot.
*Accumulation of sphingomyelin*
- This is the hallmark of **Niemann-Pick disease**, caused by **sphingomyelinase deficiency**.
- While Niemann-Pick disease also presents with a **cherry-red macular spot** and neurodegeneration, it is classically associated with **hepatosplenomegaly**, which is explicitly stated to be absent in this patient.
Question 3: A 4-month-old male infant is brought in because he rejects food and is losing weight. He had several upper respiratory tract infections during the last 2 months. Upon examination, hepatosplenomegaly is noted, as well as mild hypotonia. During the next few weeks, hepatosplenomegaly progresses, the boy fails to thrive, and he continues to reject food. He has a blood pressure of 100/70 mm Hg and heart rate of 84/min. Blood tests show pancytopenia and elevated levels of transaminases. Slit lamp examination shows bilateral cherry-red spots on the macula. Chest X-ray shows a reticulonodular pattern and calcified nodules. Biopsy of the liver shows foamy histiocytes. What is the most likely diagnosis?
A. Niemann-Pick disease type A (Correct Answer)
B. Tay-Sachs disease
C. Gaucher disease
D. Wolman disease
E. GM1 gangliosidosis
Explanation: ***Niemann-Pick disease type A***
- This presentation of a 4-month-old with **failure to thrive**, progressive **hepatosplenomegaly**, **hypotonia**, recurrent infections, **pancytopenia**, elevated transaminases, **cherry-red spots** on the macula, and **foamy histiocytes** in the liver biopsy is characteristic of Niemann-Pick disease type A.
- Niemann-Pick disease type A is a **lysosomal storage disorder** caused by a deficiency of the enzyme **sphingomyelinase**, leading to the accumulation of **sphingomyelin** in various tissues.
- The **foamy histiocytes** (lipid-laden macrophages) are a hallmark finding, and the **reticulonodular pattern** on chest X-ray represents pulmonary infiltration.
*Incorrect: Tay-Sachs disease*
- While Tay-Sachs disease also presents with **cherry-red spots** and progressive neurological deterioration in infancy, it is caused by **hexosaminidase A deficiency**.
- Key differences: Tay-Sachs typically does **not** cause **hepatosplenomegaly** or **foamy histiocytes** in the liver; the primary pathology is neuronal accumulation of GM2 ganglioside.
- Patients usually present with developmental regression, exaggerated startle response, and hypotonia, but without the prominent organomegaly seen here.
*Incorrect: Gaucher disease*
- While Gaucher disease also presents with **hepatosplenomegaly** and can cause bone marrow involvement leading to **pancytopenia**, it typically does **not** feature **cherry-red spots** or such severe early neurological regression.
- It is caused by a deficiency of **glucocerebrosidase**, leading to accumulation of glucocerebroside in **Gaucher cells** (not foamy histiocytes).
- The infantile neuronopathic form (type 2) can present early but lacks cherry-red spots.
*Incorrect: Wolman disease*
- Wolman disease is a lysosomal storage disorder caused by **lysosomal acid lipase deficiency**, presenting with hepatosplenomegaly, failure to thrive, and foamy histiocytes.
- Key distinguishing feature: **bilateral adrenal calcifications** on imaging, which are pathognomonic for Wolman disease but not mentioned in this case.
- Does **not** typically cause cherry-red spots on fundoscopic examination.
*Incorrect: GM1 gangliosidosis*
- GM1 gangliosidosis can present with hepatosplenomegaly, developmental delay, and **cherry-red spots** (in about 50% of cases).
- However, it is characterized by distinctive **coarse facial features**, **skeletal dysplasia** (dysostosis multiplex), and **vacuolated lymphocytes** on blood smear.
- The **foamy histiocytes** and prominent pulmonary involvement are more characteristic of Niemann-Pick disease type A.
Question 4: A 10-month-old boy is brought to the physician by his mother for evaluation of abnormal growth and skin abnormalities. His mother has also noticed that his eyes do not fully close when sleeping. He is at the 24th percentile for height, 17th percentile for weight, and 29th percentile for head circumference. Physical examination shows wrinkled skin, prominent veins on the scalp and extremities, and circumoral cyanosis. Genetic testing shows a point mutation in a gene that encodes for a scaffold protein of the inner nuclear membrane. The mutation causes a deformed and unstable nuclear membrane, which leads to premature aging. Which of the following is most likely to be the defective protein?
A. Vimentin
B. Lamin (Correct Answer)
C. Plectin
D. Nesprin
E. Desmin
Explanation: ***Lamin***
- The clinical presentation with **accelerated aging** symptoms (wrinkled skin, prominent veins, abnormal growth percentiles, lagophthalmos/difficulty closing eyes) combined with a defect in a **scaffold protein** of the **inner nuclear membrane** is diagnostic of **Hutchinson-Gilford Progeria Syndrome (HGPS)**.
- **Lamins** (specifically Lamin A/C) are intermediate filaments that form the **nuclear lamina**, the primary structural scaffold underlying the inner nuclear membrane, and mutations in the **LMNA gene** cause progeria and other laminopathies.
- The mutation typically produces progerin, an abnormal lamin protein that destabilizes the nuclear envelope leading to premature cellular senescence.
*Vimentin*
- **Vimentin** is an intermediate filament primarily found in **mesenchymal cells** and plays a role in cell shape, integrity, and motility within the **cytoplasm**.
- Defects in vimentin are not associated with disorders of the nuclear membrane or premature aging syndromes.
*Plectin*
- **Plectin** is a **cytoskeletal linker protein** that cross-links intermediate filaments to each other, to microtubules, and to actin filaments, reinforcing cellular stability.
- While important for cellular integrity, plectin is a **cytoplasmic protein**, not a component of the inner nuclear membrane scaffold.
*Nesprin*
- **Nesprins** (Nuclear Envelope Spectrin-repeat Proteins) are components of the **Linker of Nucleoskeleton and Cytoskeleton (LINC) complex**, bridging the nuclear lamina to the cytoskeleton at the **outer nuclear membrane**.
- While nesprins interact with the nuclear envelope, they are not the primary scaffold protein of the **inner nuclear membrane** itself (that role belongs to lamins), and mutations in nesprins are associated with muscular dystrophies, not progeria.
*Desmin*
- **Desmin** is an intermediate filament found predominantly in **muscle cells** (cardiac, skeletal, and smooth muscle), forming a scaffold that connects myofibrils to each other and to the sarcolemma.
- Mutations in desmin are associated with **myopathies** and **cardiomyopathies**, not with defects in the inner nuclear membrane or premature aging.
Question 5: A 7-month-old boy is brought to the pediatrician by his parents due to progressively worsening weakness for the last three months. The parents also describe the boy as having an exaggerated response when startled as well as diminishing response to visual stimuli. At birth, the boy was healthy and remained as such for the first few months of life. The mother says pregnancy was unremarkable, and the boy was born at 39 weeks with no complications during delivery. He is up to date on his vaccinations. The boy's grandparents immigrated from an eastern European country. Physical examination reveals hyperreflexia. Abdominal examination reveals no abnormalities. On fundoscopy, the following is seen. Which of the following is most likely deficient in this patient?
A. Arylsulfatase A
B. β-Glucosidase
C. α-Galactosidase
D. Hexosaminidase B
E. Hexosaminidase A (Correct Answer)
Explanation: ***Hexosaminidase A***
- Deficiency of **Hexosaminidase A** leads to **Tay-Sachs disease**, characterized by deterioration in motor and cognitive functions, aligning with the symptoms of weakness and abnormal responses.
- The condition is associated with a **cherry-red spot** on the retina, often observed in patients, further confirming the diagnosis.
- **Ashkenazi Jewish ancestry** is a key risk factor, consistent with the patient's eastern European heritage.
*Arylsulfatase A*
- Deficiency causes **metachromatic leukodystrophy**, which typically presents with **ataxia** and loss of previously attained skills, but not the exaggerated startle response noted in this case.
- The condition is not commonly associated with the specific visual and neurologic symptoms observed in the patient.
*β-Glucosidase*
- Deficiency leads to **Gaucher's disease**, which presents with splenomegaly, bone pain, and anemia, rather than the neurological symptoms seen here.
- The symptoms do not match the **progressive weakness** and startle reflex changes described.
*α-Galactosidase*
- Lack of this enzyme results in **Fabry disease**, which mainly causes pain episodes, skin lesions, and organ dysfunction, especially renal involvement.
- Neurological symptoms described here do not fit the typical presentation seen in Fabry disease.
*Hexosaminidase B*
- Deficiency causes **Sandhoff disease**, which presents similarly to Tay-Sachs with developmental regression and cherry-red spot.
- However, Sandhoff disease typically includes **hepatosplenomegaly**, which is notably absent in this patient on abdominal examination.
- The normal abdominal findings help distinguish this from Hexosaminidase A deficiency.
Question 6: A 1-year-old boy is brought to his pediatrician for a follow-up appointment. He was recently diagnosed with failure to thrive and developmental delay. His weight is 7 kg (15.4 lb), height is 61 cm (24 in), and head circumference is 42 cm (16.5 in). The patient’s father had a younger sister who suffered from mental and physical delay and died at a very young age. The patient was able to raise his head at the age of 7 months and began to sit alone only recently. He babbles, coos, and smiles to other people. On presentation, his blood pressure is 75/40 mm Hg, heart rate is 147/min, respiratory rate is 28/min, and temperature is 36.4°C (97.5°F). He has a coarse face with small deep orbits, proptotic eyes, big lips, and gingival hyperplasia. His skin is pale with decreased elasticity. His lung and heart sounds are normal. Abdominal examination reveals diminished anterior abdominal wall muscle tone and hepatomegaly. Muscle tone is increased in all groups of muscles on both upper and lower extremities. The physician becomes concerned and performs testing for the suspected hereditary disease. A blood test shows increased lysosomal enzyme concentration in the serum and decreased N-acetylglucosamine-1-phosphotransferase (GlcNAc phosphotransferase) activity within the leukocytes. Which of the statements listed below describes the mechanism of the patient’s condition?
A. The lysosomal enzymes are secreted from the cells instead of being targeted to lysosomes because of lack of mannose phosphorylation on N-linked glycoproteins. (Correct Answer)
B. There is impaired hydrolysis of GM2-ganglioside, which accumulates in the cytoplasm.
C. Due to enzyme deficiency, glycogen is extensively accumulated within the hepatocytes.
D. The patient’s symptoms are due to dysfunctional metabolism of sphingomyelin, which accumulates within the lysosomes.
E. The symptoms result from defective glycolysis, which results in a total energy deficiency.
Explanation: ***The lysosomal enzymes are secreted from the cells instead of being targeted to lysosomes because of lack of mannose phosphorylation on N-linked glycoproteins.***
- This describes **I-cell disease (mucolipidosis II)**, a lysosomal storage disorder characterized by a deficiency of **N-acetylglucosamine-1-phosphotransferase**.
- Without this enzyme, mannose residues on lysosomal enzymes cannot be phosphorylated, preventing their proper targeting to lysosomes and leading to their secretion into the bloodstream.
*There is impaired hydrolysis of GM2-ganglioside, which accumulates in the cytoplasm.*
- This mechanism describes **Tay-Sachs disease**, which is caused by a deficiency in B-hexosaminidase A.
- While Tay-Sachs also causes developmental delay, the clinical features and biochemical findings (specifically the lysosomal enzyme activity levels in serum and leukocytes) do not match those of the described patient.
*Due to enzyme deficiency, glycogen is extensively accumulated within the hepatocytes.*
- This mechanism describes **glycogen storage diseases**, such as Von Gierke disease (type I) or Pompe disease (type II).
- While Pompe disease is a lysosomal storage disorder, the enzymatic defect involves **acid alpha-glucosidase** and leads to glycogen accumulation primarily in muscles and heart, not related to N-acetylglucosamine-1-phosphotransferase deficiency.
*The patient’s symptoms are due to dysfunctional metabolism of sphingomyelin, which accumulates within the lysosomes.*
- This mechanism describes **Niemann-Pick disease**, caused by a deficiency in **sphingomyelinase**.
- While it is a lysosomal storage disease with hepatosplenomegaly and developmental delay, the specific enzymatic defect and the coarse facial features do not align with the patient's presentation and lab results.
*The symptoms result from defective glycolysis, which results in a total energy deficiency.*
- Defective glycolysis would lead to issues with cellular energy production, causing symptoms such as muscle weakness and fatigue.
- However, this mechanism does not explain the specific clinical features like coarse facies, gingival hyperplasia, hepatomegaly, or the characteristic enzymatic defect seen in I-cell disease.
Question 7: A scientist wants to determine if a specific fragment is contained within genome X. She uses a restriction enzyme to digest the genome into smaller fragments to run on an agarose gel, with the goal of separating the resulting fragments. A nitrocellulose blotting paper is then used to transfer the fragments from the agarose gel. A radiolabeled probe containing a complementary sequence to the fragment she is searching for is incubated with the blotting paper. Which of the following is the RNA equivalent of this technique?
A. RT-PCR
B. Western blot
C. qPCR
D. Northern blot (Correct Answer)
E. Southern blot
Explanation: **Northern blot**
- The technique described in the question, involving **restriction enzyme digestion**, **agarose gel electrophoresis**, **blotting onto a membrane**, and **hybridization with a labeled probe**, is characteristic of a **Southern blot** for DNA
- The **Northern blot** is the analogous technique used to detect and quantify **RNA** sequences, following the same principles of separation by size and detection by hybridization with a complementary probe
- Both Southern and Northern blots use the same workflow: separate nucleic acids by size on gel → transfer to membrane → detect with complementary probe
*RT-PCR*
- **Reverse transcriptase polymerase chain reaction (RT-PCR)** is used to amplify specific **RNA** sequences by first converting **RNA** into **complementary DNA (cDNA)** using reverse transcriptase, followed by standard PCR
- Unlike Northern blot, it is an **amplification technique** rather than a direct visualization method via blotting
*Western blot*
- **Western blot** is a technique used to detect and identify specific **proteins**, not nucleic acids
- It involves **gel electrophoresis** to separate proteins by size, followed by transfer to a membrane and detection using **antibodies** rather than nucleic acid probes
*qPCR*
- **Quantitative polymerase chain reaction (qPCR)**, also known as real-time PCR, is a technique used to **quantify DNA or RNA** (after reverse transcription) in real-time
- It measures the accumulation of fluorescent signal during the PCR reaction, allowing for real-time monitoring and quantification, which is fundamentally different from a blotting technique
*Southern blot*
- The description in the question *is* a **Southern blot**, which is used for **DNA** detection, not RNA
- Since the question asks for the **RNA equivalent** of the described technique, and Southern blot detects DNA, Northern blot is the correct answer
Question 8: A 70-year-old man comes to the physician because of progressive fatigue and lower back pain for the past 4 months. The back pain worsened significantly after he had a minor fall while doing yard work the previous day. For the past year, he has had a feeling of incomplete emptying of his bladder after voiding. His vital signs are within normal limits. Examination shows bilateral paravertebral muscle spasm, severe tenderness over the second lumbar vertebra, and mild tenderness over the lower thoracic vertebrae. Neurologic examination shows no abnormalities. His hemoglobin is 10.5 g/dl, alkaline phosphatase is 110 U/L, and serum calcium is 11.1 mg/dl. An x-ray of the skull is shown. Which of the following is the most appropriate next step in diagnosis?
A. Bone marrow biopsy
B. Bone scan
C. Serum vitamin D levels
D. Prostate biopsy
E. Serum protein electrophoresis (Correct Answer)
Explanation: ***Serum protein electrophoresis***
- The patient's symptoms (fatigue, back pain with minor fall causing vertebral tenderness), laboratory findings (**anemia**, **hypercalcemia**), and especially the skull X-ray showing **numerous lytic lesions** (also known as "punched-out" lesions) are highly suggestive of **multiple myeloma**.
- **Serum protein electrophoresis** (SPEP) is the most appropriate next step as it is crucial for identifying and quantifying the **monoclonal protein (M-spike)** produced by plasma cells, which is diagnostic for multiple myeloma.
*Bone marrow biopsy*
- While a bone marrow biopsy is used to confirm the diagnosis of multiple myeloma by identifying **clonal plasma cells**, it is typically done after initial screening tests like SPEP strongly suggest the diagnosis.
- SPEP is a less invasive and often the first definitive diagnostic step before proceeding to bone marrow biopsy.
*Bone scan*
- **Bone scans (technetium-99m scintigraphy)** are generally not useful for detecting the purely **lytic lesions** characteristic of multiple myeloma, as these lesions do not involve increased osteoblastic activity (bone formation) which is necessary for tracer uptake.
- X-rays and MRI are more effective for visualizing lytic lesions in multiple myeloma.
*Serum vitamin D levels*
- While vitamin D levels are important for bone health, measuring them is not a primary diagnostic step in a suspected case of multiple myeloma.
- The patient's symptoms and signs point strongly towards a neoplastic process, not a primary vitamin D deficiency.
*Prostate biopsy*
- The patient's age and urinary symptoms (incomplete bladder emptying) could raise suspicion for **prostatic enlargement** (benign prostatic hyperplasia or prostate cancer). However, the prominent lytic bone lesions, hypercalcemia, and anemia point more definitively towards a systemic hematologic malignancy like multiple myeloma rather than metastatic prostate cancer.
- While prostate cancer can cause blastic bone metastases, the X-ray shows classic lytic lesions, which are less typical of prostate cancer metastases and more characteristic of multiple myeloma.
Question 9: An 8-month-old female infant from a first-degree consanguineous couple was brought to the physician because the mother noticed abnormalities in the growth of her child as well as the different lengths of her child's legs. The infant had gingival hyperplasia, restricted movement in both shoulders, a prominent, pointed forehead, and enophthalmos with a slight opacity in both corneas. A blood test revealed 10 fold higher than normal levels of the following enzymes: N-acetyl-ß-glucosaminidase, ß-glucuronidase, ß-hexosaminidase A, and alkaline phosphatase. Which of the following is most likely deficient in this patient?
A. Lysosomal alpha-1,4-glucosidase
B. Glucose-6-phosphate dehydrogenase
C. N-acetyl-glucosamine-1-phosphotransferase (Correct Answer)
D. Glucocerebrosidase
E. Alpha-galactosidase A
Explanation: ***N-acetyl-glucosamine-1-phosphotransferase***
- The clinical presentation with **gingival hyperplasia**, **restricted joint movement**, **skeletal abnormalities** (growth abnormalities, leg length discrepancy, prominent forehead), and **corneal opacity** with elevated lysosomal enzymes (N-acetyl-ß-glucosaminidase, ß-glucuronidase, ß-hexosaminidase A) is highly characteristic of **I-cell disease** (mucolipidosis II).
- I-cell disease is caused by a deficiency in **N-acetyl-glucosamine-1-phosphotransferase**, an enzyme crucial for phosphorylating mannose residues on lysosomal enzymes, tagging them for delivery to lysosomes. Without this tag, lysosomal enzymes are secreted extracellularly, leading to their accumulation in the blood and their deficiency within lysosomes, causing the clinical features.
*Lysosomal alpha-1,4-glucosidase*
- Deficiency of **lysosomal alpha-1,4-glucosidase** causes **Pompe disease (glycogen storage disease type II)**, which is characterized by **cardiomegaly**, hypotonia, and liver involvement, but typically does not present with the skeletal dysplasias, gingival hyperplasia, or corneal clouding seen in this patient.
- While it is a lysosomal storage disorder, the specific clinical features and panel of elevated enzymes differ significantly from this case.
*Glucose-6-phosphate dehydrogenase*
- Deficiency of **glucose-6-phosphate dehydrogenase (G6PD)** causes **G6PD deficiency**, an X-linked disorder leading to **hemolytic anemia** in response to oxidative stress (e.g., fava beans, certain drugs, infections).
- It does not present with the systemic skeletal, connective tissue, and corneal abnormalities described, nor does it involve elevated lysosomal enzyme levels.
*Glucocerebrosidase*
- Deficiency of **glucocerebrosidase** causes **Gaucher disease**, which presents with **hepatosplenomegaly**, bone crises, pancytopenia, and sometimes neurological involvement.
- While it is a lysosomal storage disorder, the clinical features (e.g., absence of gingival hyperplasia, corneal opacity, or specific skeletal dysplasias like restricted joint movement) and the pattern of elevated enzymes do not match the patient's presentation.
*Alpha-galactosidase A*
- Deficiency of **alpha-galactosidase A** causes **Fabry disease**, an X-linked lysosomal storage disorder characterized by **neuropathic pain**, **angiokeratomas**, renal failure, and cardiac involvement.
- The clinical picture of Fabry disease does not include gingival hyperplasia, prominent skeletal abnormalities, or the specific pattern of elevated lysosomal enzymes observed in this patient.
Question 10: A 47-year-old homeless man is brought to the emergency department by police, who found him sleeping by the side of the street. He is somnolent and confused and is unable to give a reliable history. His medical history is unobtainable. Vital signs include: temperature 36.9°C (98.4°F), blood pressure 112/75 mm Hg, and pulse 85/min. Physical examination reveals that he has severe truncal ataxia and horizontal gaze palsy with impaired vestibulo-ocular reflexes. Muscle stretch reflexes and motor strength are normal. He has no sensory deficits. Which of the following best represents the most likely etiology of this patient’s condition?
A. Miller-Fisher syndrome
B. Vitamin B1 deficiency (Correct Answer)
C. Vitamin B12 deficiency
D. Delirium tremens
E. Ethylene glycol intoxication
Explanation: ***Vitamin B1 deficiency***
- The patient's **somnolence, confusion, truncal ataxia, and horizontal gaze palsy** are classic symptoms of **Wernicke encephalopathy**, which is caused by acute **thiamine (vitamin B1) deficiency**.
- This condition is common in individuals with **alcohol use disorder** or malnutrition, as the patient's homeless status suggests.
*Miller-Fisher syndrome*
- This is a rare variant of **Guillain-Barré syndrome** characterized by the triad of **ataxia, areflexia, and ophthalmoplegia**.
- While ophthalmoplegia and ataxia are present, the patient's **normal muscle stretch reflexes** and lack of significant **motor weakness** make this diagnosis less likely.
*Vitamin B12 deficiency*
- Leads to **subacute combined degeneration** of the spinal cord, causing **ataxia, paresthesias, weakness, and loss of proprioception and vibratory sensation**.
- The acute presentation with **gaze palsy** and the absence of sensory deficits or significant motor weakness make this less probable.
*Delirium tremens*
- Typically occurs due to **severe alcohol withdrawal** and presents with **agitation, hallucinations, tremors, and autonomic instability** (e.g., fever, tachycardia, hypertension).
- The patient's presentation of somnolence, confusion, ataxia, and gaze palsy is not typical for delirium tremens.
*Ethylene glycol intoxication*
- Can cause **neurological symptoms** like altered mental status and ataxia, and in severe cases, ophthalmoplegia due to **cranial nerve involvement**.
- However, it is also associated with **acute kidney injury, metabolic acidosis with a high anion gap, and calcium oxalate crystaluria**, none of which are indicated in the provided information.
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