USMLE Biochemistry

Question 1

Which enzyme is deficient in phenylketonuria (PKU)?

Accepted Answer

Phenylalanine hydroxylase

Answer

Tyrosinase

Answer

Homogentisate oxidase

Answer

Fumarylacetoacetate hydrolase

Question 2

A patient with jaundice, hemolytic anemia, and splenomegaly has a G6PD deficiency. Which pathway is primarily affected?

Accepted Answer

Hexose monophosphate shunt

Answer

Glycolysis

Answer

Gluconeogenesis

Answer

Citric acid cycle

Question 3

Which vitamin is a cofactor for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase?

Accepted Answer

Thiamine (B1)

Answer

Riboflavin (B2)

Answer

Niacin (B3)

Answer

Pyridoxine (B6)

Question 4

Maple syrup urine disease results from deficiency of which enzyme?

Accepted Answer

Branched-chain alpha-ketoacid dehydrogenase

Answer

Branched-chain aminotransferase

Answer

Propionyl-CoA carboxylase

Answer

Methylmalonyl-CoA mutase

Question 5

In the urea cycle, which amino acid donates the second nitrogen atom incorporated into urea?

Accepted Answer

Aspartate

Answer

Glutamate

Answer

Alanine

Answer

Glutamine

Question 6

Which enzyme catalyzes the rate-limiting step of fatty acid synthesis?

Accepted Answer

Acetyl-CoA carboxylase

Answer

Fatty acid synthase

Answer

HMG-CoA reductase

Answer

Malonyl-CoA decarboxylase