A group of scientists is studying the mechanism of action of various pancreatic hormones in rats. The scientists studied hormone A, which is secreted by the β-cells of the pancreas, and found that hormone A binds to a complex dimeric receptor on the cell membrane and exerts its effects via phosphorylation and subsequent downstream signaling that includes dephosphorylation of different intracellular proteins. Now they are studying hormone B, which is secreted by the α-cells and antagonizes the actions of hormone A. Which 2nd messenger system would hormone B utilize to exert its cellular effects?
Q52
A 7-year-old boy is brought to the physician for evaluation of developmental delay and intellectual disability. He has been admitted to the hospital twice in the past 6 months because of a cerebral venous thrombosis and a pulmonary embolism, respectively. He is at 10th percentile for weight and 95th percentile for height. Physical examination shows bilateral downward and inward subluxation of the lenses. He has a high-arched palate and kyphosis. Laboratory studies show increased serum concentration of 5-methyltetrahydrofolate. Which of the following additional findings is most likely in this patient's serum?
Q53
A 59-year-old man comes to the physician because of bilateral blurry vision and difficulty driving at night that has been worsening progressively over the past 5 months. He has hypertension, type 2 diabetes mellitus, and hyperlipidemia. His hemoglobin A1c concentration is 8.9 mg/dL. A slit-lamp shows cloudy opacities of the lenses bilaterally. The patient's eye condition is most likely due to increased activity of which of the following enzymes?
Q54
A 45-year-old woman with type 1 diabetes mellitus is brought to the emergency department by her husband because of polyuria, nausea, vomiting, and altered mental status for 4 hours. On arrival, she is unconscious. Treatment with a drug is begun that increases glucose transport to skeletal muscle and adipose tissue. Which of the following cellular events is most likely to also occur in response to this drug?
Q55
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?
Q56
A 22-year-old man comes to the physician for a routine physical examination. He feels well. He has no history of major medical illness and takes no medications. His vital signs are within normal limits. Examination shows no abnormalities. Laboratory studies, including a complete blood count and a standard electrolyte panel, are within normal limits. Urine dipstick is negative for glucose; a reducing substance test result of the urine is positive. Which of the following is the most appropriate dietary recommendation for this patient?
Q57
A 20-year-old male presents to the emergency department because of several days of back pain and fatigue. He is a college student who just returned from a study abroad program in Morocco. During his final week abroad he engaged in a number of recreational activities including swimming at the beach, eating local foods such as couscous and bean salad, and riding a camel into the desert. His temperature is 99°F (37°C), blood pressure is 121/79 mmHg, pulse is 70/min, and respirations are 11/min. He says that otherwise he has been healthy except for some episodes of dark urine. Upon physical exam, his skin is found to be more yellow than usual under his eyelids. Which of the following findings would most likely be seen in this patient?
Q58
A 3400-g (7 lb 8 oz) female newborn is delivered at 40 weeks' gestation. Physical examination shows pale skin, blonde hair, and blue irides. Her parents are from Haiti and express concern regarding the newborn's appearance. The most likely cause of this newborn's condition is a defect in which of the following processes?
Q59
A 3-day-old girl is brought to the physician by her mother because of difficulty feeding and lethargy for 1 day. She had jaundice after birth and was scheduled for a follow-up visit the next day. Her hemoglobin is 18.5 g/dL, total bilirubin is 38.1 mg/dL, and direct bilirubin is 0.1 mg/dL. Despite appropriate measures, the infant dies. At autopsy, examination of the brain shows deep yellow staining of the basal ganglia and subthalamic nuclei bilaterally. Which of the following is the most likely cause of this infant's findings?
Q60
A 59-year-old man is brought to the emergency department with signs of spontaneous bruising of the lower legs. The patient has a history of alcohol use disorder and has been unemployed for the last 2 years. He reports a 1-year history of fatigue and joint pain. Physical examination of the patient’s legs reveals the findings illustrated in the image. Oral examination shows swollen gums, petechiae of the hard palate, and poor dentition. The most likely underlying cause of this patient's current findings involves which of the following metabolic deficiencies?
Metabolism US Medical PG Practice Questions and MCQs
Question 51: A group of scientists is studying the mechanism of action of various pancreatic hormones in rats. The scientists studied hormone A, which is secreted by the β-cells of the pancreas, and found that hormone A binds to a complex dimeric receptor on the cell membrane and exerts its effects via phosphorylation and subsequent downstream signaling that includes dephosphorylation of different intracellular proteins. Now they are studying hormone B, which is secreted by the α-cells and antagonizes the actions of hormone A. Which 2nd messenger system would hormone B utilize to exert its cellular effects?
A. Direct cytoplasmic receptor binding
B. Phospholipase C
C. Tyrosine kinase
D. Direct nuclear receptor binding
E. Adenylyl cyclase-cyclic AMP (Correct Answer)
Explanation: ***Adenylyl cyclase-cyclic AMP***
- Hormone B is **glucagon**, secreted by pancreatic α-cells, which antagonizes the effects of insulin (hormone A). Glucagon primarily acts through a **G protein-coupled receptor** that activates **adenylyl cyclase**, leading to an increase in intracellular **cyclic AMP (cAMP)**.
- Increased cAMP then activates **protein kinase A (PKA)**, which phosphorylates various intracellular proteins to promote **glycogenolysis** and **gluconeogenesis**, thereby raising blood glucose levels.
*Direct cytoplasmic receptor binding*
- This mechanism is characteristic of **steroid hormones**, which are lipid-soluble and can diffuse across the cell membrane to bind to receptors in the cytoplasm.
- Pancreatic hormones like glucagon are **peptide hormones**, which are water-soluble and typically bind to cell surface receptors.
*Phospholipase C*
- Activation of **phospholipase C (PLC)** leads to the production of **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**, which mobilize intracellular calcium and activate protein kinase C, respectively.
- While some G protein-coupled receptors activate PLC, **glucagon's primary signaling pathway** involves adenylyl cyclase.
*Tyrosine kinase*
- **Tyrosine kinase receptors** are often associated with growth factors and insulin (hormone A) signaling, leading to phosphorylation of tyrosine residues on target proteins.
- Glucagon's receptor is a **G protein-coupled receptor**, not a receptor tyrosine kinase, and its actions are mediated through serine/threonine phosphorylation via PKA.
*Direct nuclear receptor binding*
- This mechanism is typical for **steroid hormones** and **thyroid hormones**, which are lipid-soluble and bind to receptors in the nucleus to directly influence gene transcription.
- As a peptide hormone, glucagon binds to cell surface receptors and does not directly interact with nuclear receptors.
Question 52: A 7-year-old boy is brought to the physician for evaluation of developmental delay and intellectual disability. He has been admitted to the hospital twice in the past 6 months because of a cerebral venous thrombosis and a pulmonary embolism, respectively. He is at 10th percentile for weight and 95th percentile for height. Physical examination shows bilateral downward and inward subluxation of the lenses. He has a high-arched palate and kyphosis. Laboratory studies show increased serum concentration of 5-methyltetrahydrofolate. Which of the following additional findings is most likely in this patient's serum?
A. Decreased methionine concentration (Correct Answer)
B. Increased propionyl-CoA concentration
C. Increased S-adenosylhomocysteine concentration
D. Decreased cystathionine concentration
E. Decreased cysteine concentration
Explanation: ***Decreased methionine concentration***
- The patient presents with **homocystinuria**, evidenced by developmental delay, intellectual disability, marfanoid habitus (tall stature, high-arched palate, kyphosis), **downward/inward lens subluxation**, and recurrent **thromboembolic events**.
- The key diagnostic finding is **increased serum 5-methyltetrahydrofolate (5-MTHF)**, which indicates a defect in **methionine synthase** or **cobalamin (vitamin B12) metabolism**, NOT MTHFR deficiency.
- **Methionine synthase** uses 5-MTHF as a methyl donor to convert homocysteine to methionine. When this enzyme is deficient (or cobalamin cofactor is unavailable), 5-MTHF accumulates (**methyl-trap hypothesis**), homocysteine increases, and **methionine decreases**.
- This biochemical pattern (↑ homocysteine, ↓ methionine, ↑ 5-MTHF) distinguishes methionine synthase deficiency from the more common **CBS deficiency**, which would show ↑ homocysteine, normal/↑ methionine, and normal/↓ 5-MTHF.
*Increased propionyl-CoA concentration*
- Elevated **propionyl-CoA** is characteristic of **propionic acidemia** (propionyl-CoA carboxylase deficiency) or **methylmalonic acidemia**.
- These organic acidemias present with metabolic acidosis, ketosis, hyperammonemia, and developmental delay, but do NOT cause recurrent thromboembolism, lens subluxation, or elevated 5-MTHF.
*Increased S-adenosylhomocysteine concentration*
- **S-adenosylhomocysteine (SAH)** accumulates when homocysteine clearance is impaired, as SAH is hydrolyzed to homocysteine and adenosine.
- While SAH may be elevated in this patient, it is a **secondary consequence** of elevated homocysteine rather than a primary diagnostic finding. The question asks for the "most likely" additional finding that helps establish the specific enzymatic defect.
*Decreased cystathionine concentration*
- **Cystathionine** is an intermediate in the **transsulfuration pathway** (homocysteine → cystathionine → cysteine) catalyzed by **cystathionine β-synthase (CBS)**.
- In **CBS deficiency** (the most common cause of homocystinuria), cystathionine would be decreased because the enzyme forming it is deficient.
- However, in **methionine synthase deficiency** (indicated by ↑ 5-MTHF), the transsulfuration pathway remains intact, so cystathionine levels would be normal or even increased as homocysteine is shunted through this alternative pathway.
*Decreased cysteine concentration*
- **Cysteine** is the end product of the transsulfuration pathway.
- Decreased cysteine is characteristic of **CBS deficiency**, where cysteine becomes a conditionally essential amino acid.
- In **methionine synthase deficiency**, the transsulfuration pathway (CBS and cystathionine γ-lyase) remains functional, so cysteine production continues normally or may even be increased due to excess homocysteine being directed through this pathway. Therefore, cysteine would NOT be decreased in this patient.
Question 53: A 59-year-old man comes to the physician because of bilateral blurry vision and difficulty driving at night that has been worsening progressively over the past 5 months. He has hypertension, type 2 diabetes mellitus, and hyperlipidemia. His hemoglobin A1c concentration is 8.9 mg/dL. A slit-lamp shows cloudy opacities of the lenses bilaterally. The patient's eye condition is most likely due to increased activity of which of the following enzymes?
A. Galactokinase
B. Aldolase B
C. Sorbitol dehydrogenase
D. Aldose reductase (Correct Answer)
E. Glucokinase
Explanation: **Aldose reductase**
- The patient's presentation of **bilateral blurry vision**, **difficulty driving at night**, and **cloudy lens opacities** in the context of poorly controlled diabetes (HbA1c 8.9%) is classic for **diabetic cataracts**.
- **Aldose reductase** is the key enzyme in the polyol pathway that converts **glucose to sorbitol**. In hyperglycemia, increased activity of this enzyme leads to **sorbitol accumulation** in lens cells, causing osmotic damage and cataract formation.
*Galactokinase*
- **Galactokinase** is involved in galactose metabolism, converting galactose to galactose-1-phosphate.
- Deficiencies in this enzyme can lead to **galactosemia** and early-onset cataracts, but this typically presents in infancy or early childhood, not in a 59-year-old with diabetes.
*Aldolase B*
- **Aldolase B** is an enzyme critical for the metabolism of fructose in the liver.
- Its deficiency causes **hereditary fructose intolerance**, leading to symptoms like hypoglycemia, jaundice, and vomiting upon fructose ingestion, which are not relevant to this patient's eye condition.
*Sorbitol dehydrogenase*
- **Sorbitol dehydrogenase** converts **sorbitol to fructose** in the polyol pathway.
- While part of the same pathway, its activity prevents sorbitol accumulation, so an *increase* in its activity would likely be protective against diabetic complications, not causative of cataracts.
*Glucokinase*
- **Glucokinase** (also known as hexokinase IV) is an enzyme that phosphorylates glucose to glucose-6-phosphate, mainly in the liver and pancreatic beta cells.
- Mutations in glucokinase can cause various forms of diabetes, but its activity is primarily involved in glucose sensing and metabolism, not directly in the pathogenesis of diabetic cataracts through increased polyol pathway flux.
Question 54: A 45-year-old woman with type 1 diabetes mellitus is brought to the emergency department by her husband because of polyuria, nausea, vomiting, and altered mental status for 4 hours. On arrival, she is unconscious. Treatment with a drug is begun that increases glucose transport to skeletal muscle and adipose tissue. Which of the following cellular events is most likely to also occur in response to this drug?
A. Dephosphorylation of fructose-1,6-bisphosphatase (Correct Answer)
B. Increased activity of acyl-CoA dehydrogenases
C. Cleavage of UDP from UDP-glucose
D. Upregulation of glucose transporter type 3 expression
E. Phosphorylation of glycogen phosphorylase kinase
Explanation: ***Dephosphorylation of fructose-1,6-bisphosphatase***
- The patient is in diabetic ketoacidosis (DKA), and the drug administered is insulin
- Insulin promotes glucose utilization and storage, which involves inhibiting gluconeogenesis through the dephosphorylation and inactivation of fructose-1,6-bisphosphatase
- This is a key regulatory mechanism by which insulin suppresses hepatic glucose production
*Increased activity of acyl-CoA dehydrogenases*
- This enzyme is crucial for fatty acid oxidation, a process that is inhibited by insulin
- In DKA, fatty acid oxidation is elevated, leading to ketone body production, but insulin treatment reduces this activity
*Cleavage of UDP from UDP-glucose*
- This reaction occurs in the synthesis of glycogen from UDP-glucose by glycogen synthase, which is activated by insulin
- While insulin stimulates glycogen synthesis, the direct cleavage of UDP from UDP-glucose is part of the synthetic process, not a primary regulatory cellular event caused by insulin in the context of DKA treatment
*Upregulation of glucose transporter type 3 expression*
- Glucose transporter type 3 (GLUT3) is primarily found in neurons and has a high affinity for glucose, with its expression generally not significantly regulated by insulin
- Insulin primarily promotes GLUT4 translocation to the cell membrane in muscle and adipose tissue to increase glucose uptake
*Phosphorylation of glycogen phosphorylase kinase*
- Phosphorylation of glycogen phosphorylase kinase activates it, subsequently activating glycogen phosphorylase and promoting glycogen breakdown (glycogenolysis)
- Insulin inhibits glycogenolysis and promotes glycogen synthesis, meaning insulin would deactivate glycogen phosphorylase kinase through dephosphorylation
Question 55: 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 56: A 22-year-old man comes to the physician for a routine physical examination. He feels well. He has no history of major medical illness and takes no medications. His vital signs are within normal limits. Examination shows no abnormalities. Laboratory studies, including a complete blood count and a standard electrolyte panel, are within normal limits. Urine dipstick is negative for glucose; a reducing substance test result of the urine is positive. Which of the following is the most appropriate dietary recommendation for this patient?
A. Eliminate galactose and lactose
B. Eliminate fructose and sucrose
C. Increase intake of ketogenic amino acids
D. Decrease purine intake
E. No changes needed (Correct Answer)
Explanation: ***No changes needed***
- The positive urine reducing substance test with negative urine glucose in an otherwise healthy individual often indicates **essential fructosuria**, a benign condition.
- Essential fructosuria is caused by a deficiency in **fructokinase** and does not require any dietary modifications.
*Eliminate galactose and lactose*
- This recommendation would be appropriate for **galactosemia**, a condition where the body cannot properly metabolize galactose.
- Galactosemia typically presents with more severe symptoms in infancy, such as **failure to thrive**, **jaundice**, and **cataracts**, which are not present here.
*Eliminate fructose and sucrose*
- This is the recommended diet for **hereditary fructose intolerance**, a more severe disorder involving a deficiency of **aldolase B**.
- Hereditary fructose intolerance would lead to symptoms like **hypoglycemia**, **vomiting**, and **liver damage** upon ingestion of fructose, which are absent in this case.
*Increase intake of ketogenic amino acids*
- This dietary approach is typically used for conditions like **pyruvate dehydrogenase complex deficiency** or **glucose transporter type 1 deficiency syndrome (Glut1 deficiency)**.
- These conditions involve metabolic crises or neurological symptoms that are not relevant to the patient's presentation.
*Decrease purine intake*
- A low-purine diet is recommended for conditions associated with **hyperuricemia**, such as **gout** or **Lesch-Nyhan syndrome**.
- There is no indication of elevated uric acid levels or symptoms related to purine metabolism in this patient.
Question 57: A 20-year-old male presents to the emergency department because of several days of back pain and fatigue. He is a college student who just returned from a study abroad program in Morocco. During his final week abroad he engaged in a number of recreational activities including swimming at the beach, eating local foods such as couscous and bean salad, and riding a camel into the desert. His temperature is 99°F (37°C), blood pressure is 121/79 mmHg, pulse is 70/min, and respirations are 11/min. He says that otherwise he has been healthy except for some episodes of dark urine. Upon physical exam, his skin is found to be more yellow than usual under his eyelids. Which of the following findings would most likely be seen in this patient?
A. Elevated urine bilirubin
B. Decreased urine urobilinogen
C. Absent urine bilirubin (Correct Answer)
D. Elevated aspartate aminotransferase
E. Conjugated hyperbilirubinemia
Explanation: ***Absent urine bilirubin***
- This patient presents with **acute hemolytic anemia**, most likely due to **G6PD deficiency** triggered by fava bean consumption (bean salad in Morocco)
- Key features: dark urine (hemoglobinuria), jaundice, back pain (hemolytic crisis), and recent exposure to oxidative stressor (fava beans)
- In **hemolytic jaundice**, there is **unconjugated (indirect) hyperbilirubinemia** from increased RBC breakdown
- **Unconjugated bilirubin** is tightly bound to albumin and **cannot be filtered by the kidneys**, so urine bilirubin is **absent**
- This distinguishes hemolytic jaundice from hepatocellular or obstructive causes where conjugated bilirubin appears in urine
*Elevated urine bilirubin*
- This finding is characteristic of **hepatocellular jaundice** (hepatitis) or **obstructive jaundice** (bile duct obstruction)
- In these conditions, **conjugated bilirubin** accumulates and is water-soluble, allowing it to be filtered into urine
- Not seen in hemolytic anemia where unconjugated bilirubin predominates
*Decreased urine urobilinogen*
- This would suggest **complete biliary obstruction** where no bilirubin reaches the intestine to be converted to urobilinogen
- In hemolytic anemia, urine urobilinogen is actually **increased** due to elevated bilirubin production and excretion into the gut
- Opposite of what would be expected in this patient
*Elevated aspartate aminotransferase*
- **AST** is a marker of **hepatocellular injury** (hepatitis, cirrhosis)
- While AST can be mildly elevated in severe hemolysis, it is not the primary finding
- The question asks for the most likely finding, and absent urine bilirubin is more specific for hemolytic jaundice
- AST would be markedly elevated in viral hepatitis, which this patient does not have
*Conjugated hyperbilirubinemia*
- In **hemolytic anemia**, the predominant finding is **unconjugated (indirect) hyperbilirubinemia**, not conjugated
- The liver conjugates bilirubin normally but is overwhelmed by the excessive production from hemolysis
- **Conjugated hyperbilirubinemia** suggests hepatocellular dysfunction or obstruction, not hemolysis
Question 58: A 3400-g (7 lb 8 oz) female newborn is delivered at 40 weeks' gestation. Physical examination shows pale skin, blonde hair, and blue irides. Her parents are from Haiti and express concern regarding the newborn's appearance. The most likely cause of this newborn's condition is a defect in which of the following processes?
A. Transfer of melanosomes to keratinocytes
B. Glycosylation of procollagen
C. Oxidation of dihydroxyphenylalanine (Correct Answer)
D. Intracellular transport of melanosomes
E. Migration of neural crest cell derivatives
Explanation: ***Oxidation of dihydroxyphenylalanine***
- The description of **pale skin**, **blonde hair**, and **blue irides** in a newborn with Haitian parents strongly suggests **oculocutaneous albinism (OCA)**.
- OCA is primarily caused by defects in **tyrosinase**, an enzyme crucial for the **oxidation of dihydroxyphenylalanine (DOPA)** in the melanin synthesis pathway.
*Transfer of melanosomes to keratinocytes*
- Defects in the transfer of **melanosomes to keratinocytes** lead to conditions like **Griscelli syndrome**, which typically presents with **silver-gray hair**, immunodeficiency, and neurological abnormalities, not the general hypopigmentation seen here.
- While there is hypopigmentation, the specific hair color and neurological features differentiate it from albinism.
*Glycosylation of procollagen*
- Abnormalities in **procollagen glycosylation** are associated with connective tissue disorders like **Ehlers-Danlos syndrome** or **osteogenesis imperfecta**.
- These conditions do not primarily manifest as **hypopigmentation** of the skin, hair, and eyes.
*Intracellular transport of melanosomes*
- Defects in **intracellular transport of melanosomes** within melanocytes can lead to certain forms of albinism, but the most common cause, particularly in Type 1 OCA, is a direct problem with melanin synthesis itself via tyrosinase.
- This is a more downstream effect compared to the direct enzymatic defect in **tyrosinase activity** for DOPA oxidation.
*Migration of neural crest cell derivatives*
- Problems with the **migration of neural crest cell derivatives** can lead to disorders like **Waardenburg syndrome**, characterized by **patchy hypopigmentation** (e.g., white forelock) and often **sensorineural hearing loss**.
- This typically involves localized areas of depigmentation, not the generalized hypopigmentation observed in the question stem.
Question 59: A 3-day-old girl is brought to the physician by her mother because of difficulty feeding and lethargy for 1 day. She had jaundice after birth and was scheduled for a follow-up visit the next day. Her hemoglobin is 18.5 g/dL, total bilirubin is 38.1 mg/dL, and direct bilirubin is 0.1 mg/dL. Despite appropriate measures, the infant dies. At autopsy, examination of the brain shows deep yellow staining of the basal ganglia and subthalamic nuclei bilaterally. Which of the following is the most likely cause of this infant's findings?
A. Increased degradation of red blood cells
B. Decreased bilirubin uptake in hepatocytes
C. Extrahepatic obliteration of the biliary tree
D. Impaired glucuronidation of bilirubin (Correct Answer)
E. Defective intracellular bilirubin transport
Explanation: ***Impaired glucuronidation of bilirubin***
- The extremely high **unconjugated bilirubin** level (total bilirubin 38.1 mg/dL, direct bilirubin 0.1 mg/dL) combined with **kernicterus** (yellow staining of basal ganglia and subthalamic nuclei) is characteristic of severe unconjugated hyperbilirubinemia, often due to impaired bilirubin conjugation.
- This clinical picture is highly suggestive of **Crigler-Najjar syndrome type 1**, a severe genetic disorder causing a near-complete absence of **UGT1A1 enzyme activity**, leading to the inability to glucuronidate bilirubin.
*Increased degradation of red blood cells*
- While increased red blood cell degradation (hemolysis) can cause unconjugated hyperbilirubinemia, the remarkably high bilirubin level and rapid progression to severe neurological findings (kernicterus) point to an additional, more profound defect in bilirubin metabolism beyond what typically results from hemolysis alone.
- The hemoglobin level of 18.5 g/dL, though on the higher side, does not strongly suggest severe anemia from hemolysis, which would typically involve a lower hemoglobin.
*Decreased bilirubin uptake in hepatocytes*
- Defective uptake of bilirubin into hepatocytes (e.g., due to mutations in OATP transporters) would lead to unconjugated hyperbilirubinemia.
- However, the severity of the hyperbilirubinemia reaching levels sufficient to cause kernicterus so rapidly often implies a more central defect in conjugation rather than just uptake.
*Extrahepatic obliteration of the biliary tree*
- **Biliary atresia** or other forms of extrahepatic biliary obstruction typically cause **conjugated (direct) hyperbilirubinemia**, as bilirubin is conjugated but cannot be excreted into the bile ducts.
- The patient's **direct bilirubin** level is very low (0.1 mg/dL), indicating primarily unconjugated hyperbilirubinemia, which rules out biliary obstruction.
*Defective intracellular bilirubin transport*
- Problems with intracellular bilirubin transport (e.g., within hepatocytes) can contribute to hyperbilirubinemia.
- However, the primary and most severe defect leading to such extreme unconjugated hyperbilirubinemia and kernicterus in infants is typically at the level of **glucuronidation**, whether due to enzyme deficiency or inhibition.
Question 60: A 59-year-old man is brought to the emergency department with signs of spontaneous bruising of the lower legs. The patient has a history of alcohol use disorder and has been unemployed for the last 2 years. He reports a 1-year history of fatigue and joint pain. Physical examination of the patient’s legs reveals the findings illustrated in the image. Oral examination shows swollen gums, petechiae of the hard palate, and poor dentition. The most likely underlying cause of this patient's current findings involves which of the following metabolic deficiencies?
A. Gamma-carboxylation of glutamic acid residues
B. Conversion of pyruvate to acetyl-CoA
C. Intestinal absorption of Ca2+ and PO43-
D. Methylation of homocysteine
E. Hydroxylation of proline and lysine residues (Correct Answer)
Explanation: ***Hydroxylation of proline and lysine residues***
- This patient exhibits classic signs of **scurvy**, caused by **Vitamin C (ascorbic acid) deficiency**.
- Vitamin C is an essential cofactor for **prolyl hydroxylase** and **lysyl hydroxylase**, enzymes that hydroxylate proline and lysine residues during collagen synthesis.
- **Hydroxyproline** and **hydroxylysine** are critical for collagen triple helix stability and cross-linking.
- The clinical findings of **petechiae**, **spontaneous bruising**, **swollen/bleeding gums**, and **perifollicular hemorrhages** result from defective collagen in blood vessels and connective tissues.
- Risk factors include **alcohol use disorder**, **poor nutrition**, and **social isolation**, all present in this patient.
*Gamma-carboxylation of glutamic acid residues*
- This process requires **Vitamin K** as a cofactor for the post-translational modification of clotting factors (II, VII, IX, X, protein C, protein S).
- Vitamin K deficiency presents with **coagulopathy** (elevated PT/INR), **ecchymoses**, and **bleeding**, but NOT the gingival and mucosal findings seen in scurvy.
- The absence of coagulation abnormalities makes this less likely.
*Conversion of pyruvate to acetyl-CoA*
- This reaction requires **Thiamine (Vitamin B1)** as a cofactor for pyruvate dehydrogenase complex.
- Thiamine deficiency causes **beriberi** (wet: heart failure; dry: peripheral neuropathy) or **Wernicke-Korsakoff syndrome** (confusion, ataxia, ophthalmoplegia).
- While common in alcoholics, it does not cause the **hemorrhagic and gingival manifestations** seen here.
*Intestinal absorption of Ca2+ and PO43-*
- This process requires **Vitamin D**, which promotes calcium and phosphate absorption in the gut.
- Vitamin D deficiency causes **rickets** (children) or **osteomalacia** (adults) with bone pain, fractures, and hypocalcemia.
- The patient's findings reflect **vascular and connective tissue fragility**, not bone mineralization defects.
*Methylation of homocysteine*
- This reaction requires **Vitamin B12** (methionine synthase cofactor) and **folate** (methyl donor).
- Deficiency causes **hyperhomocysteinemia**, leading to **megaloblastic anemia**, **neurologic symptoms** (subacute combined degeneration with B12), and increased cardiovascular risk.
- Does not cause the **hemorrhagic and gingival manifestations** characteristic of scurvy.
