Glycoproteinoses — MCQs

10 questions

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 15-year-old boy is sent from gym class with a chief complaint of severe muscle aches. In class today he was competing with his friends and therefore engaged in weightlifting for the first time. A few hours later he was extremely sore and found that his urine was red when he went to urinate. This concerned him and he was sent to the emergency department for evaluation. Upon further questioning, you learn that since childhood he has always had muscle cramps with exercise. Physical exam was unremarkable. Upon testing, his creatine kinase level was elevated and his urinalysis was negative for blood and positive for myoglobin. Thinking back to biochemistry you suspect that he may be suffering from a hereditary glycogen disorder. Given this suspicion, what would you expect to find upon examination of his cells?

A 47-year-old woman presents to her primary care physician because of pain on urination, urinary urgency, and urinary frequency for 4 days. This is the third time for her to have these symptoms over the past 7 months. She was recently treated for candidal intertrigo. Vital signs reveal a temperature of 36.7°C (98.0°F), blood pressure of 110/70 mm Hg and pulse of 75/min. Physical examination is unremarkable except for morbid obesity. Her father has type 2 diabetes complicated by end-stage chronic kidney disease. A1C is found to be 8.5%. The patient is given a prescription for her urinary symptoms. Which of the following is the best next step for this patient?

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 5-month-old boy presents with increasing weakness for the past 3 months. The patient’s mother says that the weakness is accompanied by dizziness, sweating, and vertigo early in the morning. Physical examination shows hepatomegaly. Laboratory findings show an increased amount of lactate, uric acid, and elevated triglyceride levels. Which of the following enzymes is most likely deficient in this patient?

A 12-year-old boy is brought to the physician for a well-child examination. He feels well. He has no history of serious illness. He has received all age-appropriate screenings and immunizations. His 7-year-old brother was treated for nephrotic syndrome 1 year ago. He is at 50th percentile for height and 60th percentile for weight. His temperature is 37°C (98.6°F), pulse is 90/min, and blood pressure is 96/54 mm Hg. Physical examination shows no abnormalities. Urine dipstick shows 1+ protein. A subsequent urinalysis of an early morning sample shows: Blood negative Glucose negative Protein trace Leukocyte esterase negative Nitrite negative RBC none WBC 0–1/hpf Protein/creatinine ratio 0.2 (N ≤ 0.2) Which of the following is the most appropriate next step in management?

A 9-month-old girl is brought to the physician because of a 1-month history of poor feeding and irritability. She is at the 15th percentile for height and 5th percentile for weight. Examination shows hypotonia and wasting of skeletal muscles. Cardiopulmonary examination shows no abnormalities. There is hepatomegaly. Her serum glucose is 61 mg/dL, creatinine kinase is 100 U/L, and lactic acid is within the reference range. Urine ketone bodies are elevated. Which of the following enzymes is most likely deficient in this patient?

A 13-year-old girl is brought to the physician by her mother because of a 1-month history of abnormal movements of her muscles that she cannot control. She has a younger brother with cognitive disabilities and epilepsy. Examination shows frequent, brief, involuntary contractions of the muscle groups of the upper arms, legs, and face that can be triggered by touch. An EEG shows generalized epileptiform activity. A trichrome stain of a skeletal muscle biopsy specimen shows muscle fibers with peripheral red inclusions that disrupt the normal fiber contour. Which of the following is the most likely underlying mechanism of the patient's symptoms?

An 8-year-old boy presents with recurrent infections including multiple episodes of pneumonia and diarrhea. He reports difficulty seeing in the dark. Physical examination reveals white patches on the sclera and conjunctival dryness. What is the most likely cause of these findings?

Q10

A 26-year-old woman presents to a physician for genetic counseling, because she is worried about trying to have a child. Specifically, she had 2 siblings that died young from a lysosomal storage disorder and is afraid that her own children will have the same disorder. Her background is Ashkenazi Jewish, but she says that her husband's background is mixed European heritage. Her physician says that since her partner is not of Jewish background, their chance of having a child with Niemann-Pick disease is dramatically decreased. Which of the following genetic principles best explains why there is an increased prevalence of this disease in some populations?

Glycoproteinoses 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: A 15-year-old boy is sent from gym class with a chief complaint of severe muscle aches. In class today he was competing with his friends and therefore engaged in weightlifting for the first time. A few hours later he was extremely sore and found that his urine was red when he went to urinate. This concerned him and he was sent to the emergency department for evaluation. Upon further questioning, you learn that since childhood he has always had muscle cramps with exercise. Physical exam was unremarkable. Upon testing, his creatine kinase level was elevated and his urinalysis was negative for blood and positive for myoglobin. Thinking back to biochemistry you suspect that he may be suffering from a hereditary glycogen disorder. Given this suspicion, what would you expect to find upon examination of his cells?

A. Normal glycogen structure (Correct Answer)
B. Short outer glycogen chains
C. Accumulation of glycogen in lysosomes forming dense granules
D. Glycogen without normal branching pattern
E. Absence of glycogen in muscles

Explanation: ***Normal glycogen structure*** - The patient's symptoms (exercise-induced muscle cramps, myoglobinuria, and elevated CK) are classic for **McArdle disease** (Glycogen Storage Disease Type V), caused by a deficiency in **muscle glycogen phosphorylase**. - In McArdle disease, the enzyme responsible for breaking down glycogen (glycogen phosphorylase) is deficient, but the enzymes involved in synthesizing glycogen are normal. Therefore, the **structure of glycogen is normal**, but it accumulates in muscle cells because it cannot be catabolized. *Short outer glycogen chains* - **Short outer glycogen chains** are characteristic of **Cori disease** (Glycogen Storage Disease Type III), caused by a deficiency in **debranching enzyme**. - This condition also presents with hypoglycemia and hepatomegaly, which are not described in the patient's presentation. *Accumulation of glycogen in lysosomes forming dense granules* - **Accumulation of glycogen in lysosomes** and the formation of **dense granules** is characteristic of **Pompe disease** (Glycogen Storage Disease Type II), caused by a deficiency in **lysosomal alpha-glucosidase (acid maltase)**. - Pompe disease typically presents as a severe infantile form with cardiomegaly and hypotonia, or a later-onset form with proximal muscle weakness, which differs from the patient's primary complaint of exercise intolerance and myoglobinuria. *Glycogen without normal branching pattern* - **Glycogen without a normal branching pattern** (very long unbranched chains) is characteristic of **Andersen disease** (Glycogen Storage Disease Type IV), caused by a deficiency in **branching enzyme**. - This condition typically leads to cirrhosis and liver failure in infancy, which is not consistent with the patient's presentation. *Absence of glycogen in muscles* - While McArdle disease involves an inability to break down muscle glycogen, it does not result in the **absence of glycogen** in muscles; rather, there is an **over-accumulation** of normal-structured glycogen because it cannot be utilized. - The defect is in **glycogenolysis**, not glycogen synthesis, so glycogen is formed but not broken down.

Question 3: A 47-year-old woman presents to her primary care physician because of pain on urination, urinary urgency, and urinary frequency for 4 days. This is the third time for her to have these symptoms over the past 7 months. She was recently treated for candidal intertrigo. Vital signs reveal a temperature of 36.7°C (98.0°F), blood pressure of 110/70 mm Hg and pulse of 75/min. Physical examination is unremarkable except for morbid obesity. Her father has type 2 diabetes complicated by end-stage chronic kidney disease. A1C is found to be 8.5%. The patient is given a prescription for her urinary symptoms. Which of the following is the best next step for this patient?

A. Sulphonylurea added to metformin
B. Bariatric surgery
C. Repeating the A1c test
D. Basal-bolus insulin
E. Metformin (Correct Answer)

Explanation: ***Metformin*** - The patient has symptoms suggestive of **uncontrolled type 2 diabetes**, including recurrent infections (urinary, intertrigo) and a familial history, despite an A1C of 8.5%. - **Metformin** is typically the first-line pharmacologic treatment for type 2 diabetes unless contraindicated, as it improves insulin sensitivity and reduces hepatic glucose production. *Sulphonylurea added to metformin* - While adding a sulfonylurea to metformin is an option for patients not reaching glycemic targets on metformin alone, **monotherapy with metformin** is the initial step for newly diagnosed or uncontrolled diabetes. - Sulfonylureas carry a higher risk of **hypoglycemia** compared to metformin and are generally added if metformin monotherapy is insufficient. *Bariatric surgery* - Bariatric surgery is a treatment option for **morbidly obese** individuals with type 2 diabetes, but it is typically considered after lifestyle modifications and pharmacotherapy have been attempted or when the BMI is very high (e.g., >40 or >35 with comorbidities). - It is not the immediate next step for managing newly diagnosed or uncontrolled diabetes, especially when first-line medications haven't been initiated. *Repeating the A1c test* - The elevated **A1C of 8.5%**, combined with classic symptoms of hyperglycemia (recurrent infections, polyuria from urinary symptoms), is sufficient for a diagnosis of diabetes. - Repeating the test immediately is unlikely to change the diagnosis and would delay necessary treatment initiation. *Basal-bolus insulin* - Basal-bolus insulin regimens are typically reserved for patients with very **high A1C levels** (e.g., >10%), significant symptoms of hyperglycemia, or those who have failed multiple oral hypoglycemic agents. - Given an A1C of 8.5%, initiating insulin is usually not the first pharmacologic step; metformin monotherapy is preferred.

Question 4: 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 5: A 5-month-old boy presents with increasing weakness for the past 3 months. The patient’s mother says that the weakness is accompanied by dizziness, sweating, and vertigo early in the morning. Physical examination shows hepatomegaly. Laboratory findings show an increased amount of lactate, uric acid, and elevated triglyceride levels. Which of the following enzymes is most likely deficient in this patient?

A. Hepatic glycogen phosphorylase
B. Debranching enzyme
C. Glucose-6-phosphatase (Correct Answer)
D. Muscle glycogen phosphorylase
E. Lysosomal α-1,4-glucosidase

Explanation: ***Glucose-6-phosphatase*** - The constellation of **hypoglycemia** (weakness, dizziness, sweating, vertigo, especially early morning), **hepatomegaly**, **lactic acidosis**, **hyperuricemia**, and **hypertriglyceridemia** are classic features of **Type I glycogen storage disease (von Gierke disease)**, which is caused by a deficiency of **glucose-6-phosphatase**. - This enzyme is crucial for the final step of both **glycogenolysis** and **gluconeogenesis**, releasing free glucose into the bloodstream; its deficiency leads to an inability to maintain normal blood glucose levels during fasting and accumulation of glucose-6-phosphate, which shunts into other metabolic pathways. *Hepatic glycogen phosphorylase* - Deficiency in **hepatic glycogen phosphorylase** (Type VI glycogen storage disease, Hers disease) would cause **hepatomegaly** and **hypoglycemia**, but typically does not present with severe **lactic acidosis**, **hyperuricemia**, or **hypertriglyceridemia** to the same degree as von Gierke disease. - The primary defect is in breaking down glycogen, leading to its accumulation in the liver, but the products of glycolysis can still exit the liver via gluconeogenesis. *Debranching enzyme* - Deficiency in **debranching enzyme** (Type III glycogen storage disease, Cori or Forbes disease) causes **hepatomegaly** and **hypoglycemia**, but usually presents with milder symptoms and less severe **lactic acidosis**, **hyperuricemia**, and **hypertriglyceridemia**. - Patients often present with symptoms similar to Type I, but muscle involvement is also common, and **glycogen structures with short outer branches** are characteristic. *Muscle glycogen phosphorylase* - Deficiency in **muscle glycogen phosphorylase** (Type V glycogen storage disease, McArdle disease) primarily affects **skeletal muscle**, leading to exercise intolerance, muscle pain, and myoglobinuria. - It does not typically cause **hypoglycemia** or **hepatomegaly**, as the liver enzyme is functional, and the symptoms described are systemic rather than muscle-specific. *Lysosomal α-1,4-glucosidase* - Deficiency in **lysosomal α-1,4-glucosidase** (Type II glycogen storage disease, Pompe disease) primarily affects the **heart, muscle, and liver**, causing severe **cardiomyopathy**, hypotonia, and **hepatomegaly**. - While it involves glycogen accumulation, it typically does not present with **hypoglycemia** (as cytoplasmic glycogen metabolism is intact), **lactic acidosis**, or the specific metabolic derangements seen in this patient.

Question 6: A 12-year-old boy is brought to the physician for a well-child examination. He feels well. He has no history of serious illness. He has received all age-appropriate screenings and immunizations. His 7-year-old brother was treated for nephrotic syndrome 1 year ago. He is at 50th percentile for height and 60th percentile for weight. His temperature is 37°C (98.6°F), pulse is 90/min, and blood pressure is 96/54 mm Hg. Physical examination shows no abnormalities. Urine dipstick shows 1+ protein. A subsequent urinalysis of an early morning sample shows: Blood negative Glucose negative Protein trace Leukocyte esterase negative Nitrite negative RBC none WBC 0–1/hpf Protein/creatinine ratio 0.2 (N ≤ 0.2) Which of the following is the most appropriate next step in management?

A. Anti-nuclear antibody level
B. Repeat urine dipstick in 1 year (Correct Answer)
C. Measure serum creatinine and urea nitrogen
D. 24-hour urine protein collection
E. Lipid profile

Explanation: ***Repeat urine dipstick in 1 year*** - The initial 1+ protein on a random dipstick with a normal early morning protein/creatinine ratio (0.2, which is ≤0.2) is consistent with **benign orthostatic (postural) proteinuria** in an otherwise healthy adolescent. - **Orthostatic proteinuria** is a common, benign finding in adolescents where protein appears in upright position but resolves when supine (hence the trace protein in early morning sample). - Given the patient is asymptomatic, has normal vital signs, no edema, and a normal protein/creatinine ratio, this represents a benign condition requiring only monitoring with annual urine dipstick to ensure no progression. - The family history of nephrotic syndrome in his brother does not change management in this asymptomatic patient with normal findings. *Anti-nuclear antibody level* - An **ANA level** is used to screen for **systemic lupus erythematosus** (SLE) and other autoimmune diseases that can cause glomerulonephritis. - The patient has no symptoms or signs (e.g., rash, joint pain, serositis, hematuria) suggestive of an autoimmune condition, and the normal protein/creatinine ratio makes significant renal pathology unlikely. *Measure serum creatinine and urea nitrogen* - This would be appropriate if there was evidence of **renal dysfunction**, such as persistent abnormal proteinuria (protein/creatinine ratio >0.2), elevated blood pressure, or other signs of kidney disease. - The patient has normal blood pressure, no edema, and a normal protein/creatinine ratio indicating no impaired kidney function. *24-hour urine protein collection* - A **24-hour urine collection** is indicated when there is sustained, significant proteinuria that needs quantification (e.g., to confirm nephrotic syndrome with >3.5 g/day or to monitor chronic kidney disease). - The patient's normal protein/creatinine ratio (0.2) already provides adequate quantification and confirms the initial 1+ protein was transient orthostatic proteinuria, making 24-hour collection unnecessary. *Lipid profile* - A **lipid profile** is typically ordered in cases of suspected or confirmed **nephrotic syndrome**, as dyslipidemia is a common complication. - This patient does not have nephrotic syndrome, which requires proteinuria >3.5 g/day (protein/creatinine ratio >3.5), hypoalbuminemia, edema, and hyperlipidemia—none of which are present.

Question 7: A 9-month-old girl is brought to the physician because of a 1-month history of poor feeding and irritability. She is at the 15th percentile for height and 5th percentile for weight. Examination shows hypotonia and wasting of skeletal muscles. Cardiopulmonary examination shows no abnormalities. There is hepatomegaly. Her serum glucose is 61 mg/dL, creatinine kinase is 100 U/L, and lactic acid is within the reference range. Urine ketone bodies are elevated. Which of the following enzymes is most likely deficient in this patient?

A. Glucose-6-phosphatase
B. Muscle phosphorylase
C. Acid alpha-glucosidase
D. Glycogen debrancher (Correct Answer)
E. Glucocerebrosidase

Explanation: ***Glycogen debrancher*** - The patient's symptoms of **hepatomegaly**, **hypoglycemia**, **poor feeding**, **growth failure**, and **elevated urine ketones** in the presence of normal lactic acid suggest Type III glycogen storage disease (Cori disease), caused by a deficiency in **glycogen debrancher enzyme**. - **Muscle wasting** and **hypotonia** are also consistent with Type III GSD, as the debranching enzyme is present in both liver and muscle. *Glucose-6-phosphatase* - Deficiency in **glucose-6-phosphatase** (Type I GSD, Von Gierke disease) also presents with **hepatomegaly** and **hypoglycemia**. - However, Type I GSD is characterized by **lactic acidosis**, which is explicitly stated as normal in this patient, and **hyperlipidemia**, which is not mentioned. *Muscle phosphorylase* - Deficiency in **muscle phosphorylase** (Type V GSD, McArdle disease) primarily affects skeletal muscle, causing **exercise intolerance** and **muscle pain**. - It does not typically present with **hypoglycemia**, **hepatomegaly**, or **growth failure** in infancy. *Acid alpha-glucosidase* - Deficiency in **acid alpha-glucosidase** (Type II GSD, Pompe disease) causes accumulation of glycogen in lysosomes, leading to severe **cardiomyopathy**, **hypotonia**, and **muscle weakness**. - While hypotonia is present, the absence of **cardiomegaly** and significant **liver involvement** makes this diagnosis less likely. *Glucocerebrosidase* - Deficiency in **glucocerebrosidase** causes Gaucher disease, a lysosomal storage disorder, not a glycogen storage disorder. - Symptoms include **hepatosplenomegaly**, **bone crises**, and neurological symptoms, but not **hypoglycemia** or isolated muscle wasting directly related to glycogen metabolism.

Question 8: A 13-year-old girl is brought to the physician by her mother because of a 1-month history of abnormal movements of her muscles that she cannot control. She has a younger brother with cognitive disabilities and epilepsy. Examination shows frequent, brief, involuntary contractions of the muscle groups of the upper arms, legs, and face that can be triggered by touch. An EEG shows generalized epileptiform activity. A trichrome stain of a skeletal muscle biopsy specimen shows muscle fibers with peripheral red inclusions that disrupt the normal fiber contour. Which of the following is the most likely underlying mechanism of the patient's symptoms?

A. CTG trinucleotide repeat expansion
B. Mutation of the methyl-CpG binding protein 2 gene
C. Truncated dystrophin protein
D. Autoimmune endomysial destruction
E. Defective oxidative phosphorylation (Correct Answer)

Explanation: ***Defective oxidative phosphorylation*** - The constellation of **uncontrolled muscle movements (myoclonus)**, **epilepsy**, and the brother's **cognitive disabilities** strongly suggests a **mitochondrial disorder**. - **Ragged red fibers** on trichrome stain of skeletal muscle biopsy are pathognomonic for **mitochondrial myopathies**, indicating defective oxidative phosphorylation due to abnormal mitochondrial aggregates. *CTG trinucleotide repeat expansion* - This is characteristic of **Myotonic Dystrophy**, which primarily presents with **myotonia** (delayed muscle relaxation), progressive muscle weakness, and often cataracts, rather than prominent myoclonus and seizures. - While muscle weakness can occur, the specific biopsy findings and prominent myoclonus point away from this diagnosis. *Mutation of the methyl-CpG binding protein 2 gene* - A mutation in the **MECP2 gene** causes **Rett Syndrome**, an X-linked dominant disorder seen almost exclusively in girls. - It involves normal development for 6-18 months followed by regression, loss of purposeful hand movements, **stereotypical hand-wringing**, and microcephaly, which are not described here. *Truncated dystrophin protein* - A truncated dystrophin protein causes **Duchenne Muscular Dystrophy**, an X-linked recessive disorder leading to progressive muscle weakness, **Gowers' sign**, and elevated creatine kinase. - This condition does not typically present with myoclonus or the characteristic ragged red fibers, nor does it typically involve the sibling's intellectual disability and epilepsy in this manner. *Autoimmune endomysial destruction* - This mechanism is characteristic of **celiac disease**, which can have neurological symptoms like ataxia or peripheral neuropathy, but not typically the severe myoclonus, epilepsy, or muscle biopsy findings seen here. - **Inflammatory myopathies** like polymyositis may show endomysial inflammation, but the clinical picture and specific biopsy findings (ragged red fibers) are not consistent.

Question 9: An 8-year-old boy presents with recurrent infections including multiple episodes of pneumonia and diarrhea. He reports difficulty seeing in the dark. Physical examination reveals white patches on the sclera and conjunctival dryness. What is the most likely cause of these findings?

A. Congenital rubella
B. Spinocerebellar ataxia (SCA) type 1
C. Deficiency of vitamin A (Correct Answer)
D. Autoimmune neutropenia
E. Vitamin B1 deficiency

Explanation: ***Deficiency of vitamin A*** - The combination of **recurrent infections** (pneumonia, diarrhea), **night blindness**, and **ocular signs** like white patches on the sclera (Bitot's spots due to keratinized epithelium) and conjunctival dryness (xerophthalmia) are classic manifestations of **vitamin A deficiency**. - Vitamin A is crucial for **immune function**, **vision** (a component of rhodopsin), and the **maintenance of epithelial tissues**. *Congenital rubella* - This typically presents with a **triad of cataracts**, **heart defects** (e.g., patent ductus arteriosus), and **sensorineural hearing loss**, often detected earlier in infancy. - It does not explain the recurrent infections, night blindness, or specific ocular epithelial changes described. *Spinocerebellar ataxia (SCA) type 1* - This is a **neurodegenerative disorder** primarily affecting coordination and balance, leading to progressive **ataxia**. - It does not cause recurrent infections, night blindness, or xerophthalmia; its symptoms are distinct and neurological in nature. *Autoimmune neutropenia* - Primarily causes **recurrent bacterial infections** due to low neutrophil counts. - It does not explain the night blindness, diarrhea, or the specific ocular findings of keratinization. *Vitamin B1 deficiency* - Also known as **thiamine deficiency**, it leads to **beriberi**, characterized by cardiac (wet beriberi) or neurological (dry beriberi) symptoms. - Common symptoms include **peripheral neuropathy**, **heart failure**, and **Wernicke-Korsakoff syndrome**, none of which are descriptive of this patient's presentation.

Question 10: A 26-year-old woman presents to a physician for genetic counseling, because she is worried about trying to have a child. Specifically, she had 2 siblings that died young from a lysosomal storage disorder and is afraid that her own children will have the same disorder. Her background is Ashkenazi Jewish, but she says that her husband's background is mixed European heritage. Her physician says that since her partner is not of Jewish background, their chance of having a child with Niemann-Pick disease is dramatically decreased. Which of the following genetic principles best explains why there is an increased prevalence of this disease in some populations?

A. Natural selection
B. Imprinting
C. De novo mutations
D. Gene flow
E. Founder effect (Correct Answer)

Explanation: ***Founder effect*** - The **founder effect** occurs when a new population is established by a small number of individuals, leading to a **reduced genetic diversity** and an increased frequency of certain alleles that were present in the founders. This is particularly relevant in populations like **Ashkenazi Jews**, who descended from a small, isolated group with certain allele frequencies. - In this scenario, the high prevalence of **Niemann-Pick disease** (and other genetic disorders) in the Ashkenazi Jewish population is due to their historical isolation and intermarriage within a relatively small gene pool, trapping and concentrating certain alleles. *Natural selection* - **Natural selection** typically describes the process by which traits that enhance survival and reproduction become more common in a population over time, or deleterious traits become less common. - While it can influence disease prevalence, it doesn't primarily explain the disproportionately high frequency of rare recessive disorders in specific isolated populations in the manner described. *Imprinting* - **Genomic imprinting** refers to the phenomenon where certain genes are expressed in a **parent-of-origin-specific manner**, meaning that only the allele inherited from either the mother or the father is expressed. - This mechanism explains certain genetic conditions but does not account for the increased prevalence of a recessive disorder due to population history and isolation. *De novo mutations* - **De novo mutations** are new genetic alterations that appear for the first time in an individual and are not inherited from either parent. - While de novo mutations are a source of genetic variation, they do not explain the high prevalence of a specific ancestral allele within an entire population. *Gene flow* - **Gene flow** (or migration) is the transfer of genetic material from one population to another, which tends to **decrease genetic differences** between populations and introduce new alleles. - This principle would suggest a *reduction* in the prevalence of specific rare alleles over time as populations mix, rather than an *increase* in isolated groups.

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