USMLE Biochemistry 3

Question 1

Which enzyme deficiency causes cystinuria by impairing renal tubular reabsorption of dibasic amino acids?

Accepted Answer

Defective cystine transporter (rBAT)

Answer

Cystathionine beta-synthase

Answer

Gamma-cystathionase

Answer

Cystine reductase

Question 2

A patient on a high-fat diet has elevated chylomicrons and VLDL. Which enzyme is responsible for clearing chylomicrons from the blood?

Accepted Answer

Lipoprotein lipase

Answer

Hepatic lipase

Answer

Hormone-sensitive lipase

Answer

LCAT

Question 3

Which step in heme synthesis occurs in the mitochondria?

Accepted Answer

Formation of delta-aminolevulinic acid (ALA) from glycine and succinyl-CoA

Answer

Conversion of porphobilinogen to hydroxymethylbilane

Answer

Formation of uroporphyrinogen III

Answer

Conversion of coproporphyrinogen to protoporphyrin

Question 4

In DNA replication, which enzyme removes the RNA primer and replaces it with DNA?

Accepted Answer

DNA polymerase I

Answer

DNA polymerase III

Answer

DNA ligase

Answer

Primase

Question 5

Which amino acid is the primary gluconeogenic substrate released from skeletal muscle during fasting?

Accepted Answer

Alanine

Answer

Glutamine

Answer

Glycine

Answer

Serine

Question 6

Orotic aciduria with megaloblastic anemia but no hyperammonemia suggests deficiency of which enzyme?

Accepted Answer

UMP synthase (UMPS)

Answer

Carbamoyl phosphate synthetase II

Answer

Ornithine transcarbamylase

Answer

Dihydroorotase