USMLE Biochemistry

Question 1

Which enzyme is deficient in phenylketonuria (PKU)?

Accepted Answer

Phenylalanine hydroxylase

Answer

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?

Accepted Answer

Hexose monophosphate shunt

Answer

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?

Accepted Answer

Thiamine (B1)

Answer

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?

Accepted Answer

Branched-chain alpha-ketoacid dehydrogenase

Answer

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?

Accepted Answer

Aspartate

Answer

Aspartate donates the second nitrogen to form argininosuccinate in the urea cycle, ultimately contributing one nitrogen to urea.

USMLE Practice Test

USMLE Practice Test

USMLE Biochemistry