USMLE Biochemistry 2 — Questions and Answers
Question 1: A child presents with megaloblastic anemia and homocystinuria but normal methylmalonic acid levels. Which vitamin deficiency is most likely?
- Folate (Correct answer)
- Vitamin B12
- Vitamin B6
- Vitamin B2
Correct answer: Folate
Folate deficiency causes megaloblastic anemia and elevated homocysteine but does not cause methylmalonic aciduria, distinguishing it from B12 deficiency.
Question 2: Which glycogen storage disease presents with hypoglycemia, lactic acidosis, and hepatomegaly due to glucose-6-phosphatase deficiency?
- Von Gierke disease (Type I) (Correct answer)
- Pompe disease (Type II)
- Cori disease (Type III)
- McArdle disease (Type V)
Correct answer: Von Gierke disease (Type I)
Von Gierke disease (Type Ia) results from glucose-6-phosphatase deficiency, preventing glucose release from the liver and causing hypoglycemia with lactic acidosis.
Question 3: In the citric acid cycle, which reaction is catalyzed by isocitrate dehydrogenase?
- Isocitrate → alpha-ketoglutarate + CO2 (Correct answer)
- Citrate → isocitrate
- Alpha-ketoglutarate → succinyl-CoA
- Succinate → fumarate
Correct answer: Isocitrate → alpha-ketoglutarate + CO2
Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate, producing CO2 and NADH.
Question 4: Alkaptonuria is caused by a deficiency of which enzyme?
- Homogentisate oxidase (Correct answer)
- Phenylalanine hydroxylase
- Tyrosine aminotransferase
- Fumarylacetoacetate hydrolase
Correct answer: Homogentisate oxidase
Alkaptonuria results from homogentisate oxidase deficiency, causing accumulation of homogentisic acid, which causes dark urine and ochronosis.
Question 5: Which lipoprotein is primarily responsible for reverse cholesterol transport?
- HDL (Correct answer)
- LDL
- VLDL
- Chylomicrons
Correct answer: HDL
HDL mediates reverse cholesterol transport by collecting cholesterol from peripheral tissues and delivering it to the liver for excretion.
Question 6: Lesch-Nyhan syndrome is caused by deficiency of which enzyme?
- HGPRT (hypoxanthine-guanine phosphoribosyltransferase) (Correct answer)
- Adenosine deaminase
- Purine nucleoside phosphorylase
- Xanthine oxidase
Correct answer: HGPRT (hypoxanthine-guanine phosphoribosyltransferase)
Lesch-Nyhan syndrome results from HGPRT deficiency, impairing purine salvage and leading to gout, intellectual disability, and self-mutilation.
USMLE Biochemistry 2
A child presents with megaloblastic anemia and homocystinuria but normal methylmalonic acid levels.
Which vitamin deficiency is most likely?