Biochemistry Flashcards
6 cards from real USMLE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Biochemistry flashcards as text
Which enzyme deficiency causes cystinuria by impairing renal tubular reabsorption of dibasic amino acids?
Answer: Defective cystine transporter (rBAT)
Cystinuria is caused by a defective cystine/dibasic amino acid transporter (rBAT) in renal tubules and intestine, leading to cystine stones.
A patient on a high-fat diet has elevated chylomicrons and VLDL. Which enzyme is responsible for clearing chylomicrons from the blood?
Answer: Lipoprotein lipase
Lipoprotein lipase on capillary endothelium hydrolyzes triglycerides in chylomicrons and VLDL, releasing fatty acids for peripheral uptake.
Which step in heme synthesis occurs in the mitochondria?
Answer: Formation of delta-aminolevulinic acid (ALA) from glycine and succinyl-CoA
The first step, formation of ALA from glycine and succinyl-CoA by ALA synthase, occurs in the mitochondrial matrix.
In DNA replication, which enzyme removes the RNA primer and replaces it with DNA?
Answer: DNA polymerase I
DNA polymerase I has 5'→3' exonuclease activity to remove RNA primers and 5'→3' polymerase activity to fill the gaps with DNA.
Which amino acid is the primary gluconeogenic substrate released from skeletal muscle during fasting?
Answer: Alanine
Alanine is the primary gluconeogenic amino acid exported from skeletal muscle to the liver, representing the glucose-alanine cycle.
Orotic aciduria with megaloblastic anemia but no hyperammonemia suggests deficiency of which enzyme?
Answer: UMP synthase (UMPS)
UMP synthase deficiency impairs pyrimidine synthesis, causing orotic acid accumulation and megaloblastic anemia without urea cycle disruption or hyperammonemia.