USMLE Biochemistry 1 — Questions and Answers
Question 1: Which enzyme is deficient in phenylketonuria (PKU)?
- Phenylalanine hydroxylase (Correct answer)
- Tyrosinase
- Homogentisate oxidase
- Fumarylacetoacetate hydrolase
Correct answer: Phenylalanine hydroxylase
PKU is caused by deficiency of phenylalanine hydroxylase, which converts phenylalanine to tyrosine.
Question 2: A patient with jaundice, hemolytic anemia, and splenomegaly has a G6PD deficiency. Which pathway is primarily affected?
- Hexose monophosphate shunt (Correct answer)
- Glycolysis
- Gluconeogenesis
- Citric acid cycle
Correct answer: Hexose monophosphate shunt
G6PD deficiency impairs the hexose monophosphate shunt, reducing NADPH and glutathione regeneration needed to protect RBCs from oxidative damage.
Question 3: Which vitamin is a cofactor for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase?
- Thiamine (B1) (Correct answer)
- Riboflavin (B2)
- Niacin (B3)
- Pyridoxine (B6)
Correct answer: Thiamine (B1)
Thiamine (B1) as thiamine pyrophosphate (TPP) is essential for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase reactions.
Question 4: Maple syrup urine disease results from deficiency of which enzyme?
- Branched-chain alpha-ketoacid dehydrogenase (Correct answer)
- Branched-chain aminotransferase
- Propionyl-CoA carboxylase
- Methylmalonyl-CoA mutase
Correct answer: Branched-chain alpha-ketoacid dehydrogenase
Maple syrup urine disease is caused by deficiency of branched-chain alpha-ketoacid dehydrogenase, leading to accumulation of leucine, isoleucine, and valine.
Question 5: In the urea cycle, which amino acid donates the second nitrogen atom incorporated into urea?
- Aspartate (Correct answer)
- Glutamate
- Alanine
- Glutamine
Correct answer: Aspartate
Aspartate donates the second nitrogen to form argininosuccinate in the urea cycle, ultimately contributing one nitrogen to urea.
Question 6: Which enzyme catalyzes the rate-limiting step of fatty acid synthesis?
- Acetyl-CoA carboxylase (Correct answer)
- Fatty acid synthase
- HMG-CoA reductase
- Malonyl-CoA decarboxylase
Correct answer: Acetyl-CoA carboxylase
Acetyl-CoA carboxylase catalyzes the conversion of acetyl-CoA to malonyl-CoA, which is the rate-limiting step of fatty acid synthesis.
USMLE Biochemistry 1
Which enzyme is deficient in phenylketonuria (PKU)?