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

Cystinuria is caused by a defective cystine/dibasic amino acid transporter (rBAT) in renal tubules and intestine, leading to cystine stones.

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

Lipoprotein lipase on capillary endothelium hydrolyzes triglycerides in chylomicrons and VLDL, releasing fatty acids for peripheral uptake.

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

The first step, formation of ALA from glycine and succinyl-CoA by ALA synthase, occurs in the mitochondrial matrix.

Question 4

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

Accepted Answer

DNA polymerase I

Answer

DNA polymerase I has 5'→3' exonuclease activity to remove RNA primers and 5'→3' polymerase activity to fill the gaps with DNA.

Question 5

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

Accepted Answer

Alanine

Answer

Alanine is the primary gluconeogenic amino acid exported from skeletal muscle to the liver, representing the glucose-alanine cycle.

USMLE Practice Test