Biochemistry Flashcards
7 cards from real NBME practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Biochemistry flashcards as text
A 6-month-old develops cataracts, jaundice, intellectual disability, and E. coli sepsis after starting milk feeds. Which enzyme is most likely deficient?
Answer: Galactose-1-phosphate uridylyltransferase (GALT)
Classic galactosemia from GALT deficiency leads to galactose-1-phosphate accumulation, causing liver damage, cataracts, and increased susceptibility to E. coli sepsis.
A toddler develops hypoglycemia, jaundice, and vomiting after eating fruit. Fructose-1-phosphate accumulates in the liver. The deficient enzyme is:
Answer: Aldolase B
Hereditary fructose intolerance is caused by aldolase B deficiency, trapping fructose-1-phosphate in the liver and depleting free phosphate and ATP.
A patient with familial hypercholesterolemia has markedly elevated LDL. The primary defect impairs which cellular process?
Answer: Receptor-mediated endocytosis of LDL particles
Familial hypercholesterolemia is caused by loss-of-function mutations in the LDL receptor, impairing endocytosis and clearance of circulating LDL.
Which is the rate-limiting enzyme of cholesterol biosynthesis and the target of statin drugs?
Answer: HMG-CoA reductase
HMG-CoA reductase converts HMG-CoA to mevalonate; it is the committed, rate-limiting step targeted by statins to lower cholesterol.
Pyruvate dehydrogenase complex deficiency results in an inability to convert pyruvate to acetyl-CoA, leading to accumulation of which metabolites?
Answer: Pyruvate, lactate, and alanine
When PDC is deficient, pyruvate cannot enter the TCA cycle and instead accumulates along with lactate (via LDH) and alanine (via transamination), causing lactic acidosis.
Which enzyme is unique to gluconeogenesis and is not used in glycolysis?
Answer: Phosphoenolpyruvate carboxykinase (PEPCK)
PEPCK converts oxaloacetate to PEP, bypassing the irreversible pyruvate kinase reaction of glycolysis; it is exclusive to gluconeogenesis.
A child with hepatomegaly, severe fasting hypoglycemia, and lactic acidosis is diagnosed with von Gierke disease (GSD type I). The deficient enzyme is:
Answer: Glucose-6-phosphatase
Glucose-6-phosphatase deficiency prevents release of free glucose from the liver during fasting, causing hypoglycemia and accumulation of glucose-6-phosphate that is shunted to glycolysis and lactic acid production.