USMLE Biochemistry Exam Questions
with All Correct Answers
What might be defective in a patient with recurrent renal staghorn caliculi? What amino
acids are affected? - Answer-defect of renal tubular amino acid transporter for cysteine,
ornithine, lysine, and arginine
What is deficient in a patient whose urine smells like maple syrup? What process
cannot be completed? - Answer-deficient alpha-ketoacid dehydroenase prevents the
degradation of branched amino acids, such as isoleucine, leucine, and valine
What is deficient in a patient who experiences painful muscle cramps and myglobinuria
with strenuous exercise? What is the name of this disease? - Answer-McArdle's disease
(type V): muscle glycogen phosphorylase deficiency
What is deficient in an infant who dies shortly after birth with cardiomegaly? What is the
name of this disease? - Answer-Pompe disease (type II, infantile form): alpha-1,4-
glucosidase deficiency
What is deficient in a patient with normal blood lactate levels with hepatomegaly,
hypoglycemia, and hyperlipidemia? What is the name of this disease? - Answer-Cori's
disease (type III): debranching enzyme (alpha-1,6-glucosidase) deficiency
What is deficient in a patient with diaphragm weakness, respiratory failure, and
cardiomegaly? What is the name of this disease? - Answer-Pompe disease (type II,
adult form): alpha-1,4-glucosidase deficiency
What is deficient in a patient with increased normal glycogen in the liver, elevated lactic
acid in the blood, hepatomegaly, hyperuricemia, and enlarged kidneys? What is the
name of this disease? - Answer-von Gierke's disease (type I): glucose-6-phosphatase
deficiency
In what part of the cell does the rate-limiting step of fatty acid synthesis synthesis
occur? - Answer-cytoplasm (acetyl CoA carboxylase)
Where in the cell does beta-oxidation occur? - Answer-mitochondrial matrix
In alcoholism, oxaloacetate is shuttled to malate due to excess of what? - Answer-
NADH
What is detected in the urine of a patient with ketoacidosis? - Answer-acetoacetate
, Between meals, what is the major source of glucose? - Answer-hepatic glycogenolysis
What are the sources of ATP in the first three days of starvation? - Answer-hepatic
glycogenosis
adipose release of free fatty acids
What triacylglycerol components can contribute to gluconeogenesis? - Answer-glycerol
and propionyl-CoA
In the first three days of starvation, what amino acids can be used in hepatic
gluconeogenesis? - Answer-lactate and alanine
What is the main source of fuel in muscles and the liver in the first three days of
starvation? - Answer-free fatty acids
What is the main source of energy in the brain and red blood cells after 3 days of
starvation? - Answer-ketone bodies
What determines survival time in starvation? - Answer-amount of adipose stores
What is the suspected diagnosis in an 2-month-old infant who displays failure to thrive,
steatorrhea, acanthocytosis, ataxia, and night blindness; intestinal biopsy shows lipid
accumulation within enterocytes? What is the inherited mutation? - Answer-
abetalipoproteinemia: mutation in the microsomal triglyceride transfer protein (MTP)
gene, leading to decreased production of B-48 and B-100
What enzyme is deficient in an infant who presents with lethargy, vomiting, and
hypotonia; labs reveal metabolic acidosis with large anion gap and elevated propionic
acid, ketosis, and hypoglycemia? - Answer-propionyl CoA carboxylase
What is the rate-limiting enzyme of glycolysis? - Answer-phosphofructokinase-1 (PFK-1)
What is the rate-limiting enzyme of gluconeogenesis? - Answer-fructose-1,6-
bisphosphatase
What is the rate-limiting enzyme of the TCA cycle? - Answer-isocitrate dehydrogenase
What is the rate-limiting enzyme of glycogen synthesis? - Answer-glycogen synthase
What is the rate-limiting enzyme of glycogenolysis? - Answer-glycogen phosphorylase
What is the rate-limiting enzyme of the HMP shunt? - Answer-glucose-6-phosphate
dehydrogenase (G6PD)
What is the rate-limiting enzyme of de novo pyrimidine synthesis? - Answer-carbamoyl
phosphate synthetase II (CPS II)
with All Correct Answers
What might be defective in a patient with recurrent renal staghorn caliculi? What amino
acids are affected? - Answer-defect of renal tubular amino acid transporter for cysteine,
ornithine, lysine, and arginine
What is deficient in a patient whose urine smells like maple syrup? What process
cannot be completed? - Answer-deficient alpha-ketoacid dehydroenase prevents the
degradation of branched amino acids, such as isoleucine, leucine, and valine
What is deficient in a patient who experiences painful muscle cramps and myglobinuria
with strenuous exercise? What is the name of this disease? - Answer-McArdle's disease
(type V): muscle glycogen phosphorylase deficiency
What is deficient in an infant who dies shortly after birth with cardiomegaly? What is the
name of this disease? - Answer-Pompe disease (type II, infantile form): alpha-1,4-
glucosidase deficiency
What is deficient in a patient with normal blood lactate levels with hepatomegaly,
hypoglycemia, and hyperlipidemia? What is the name of this disease? - Answer-Cori's
disease (type III): debranching enzyme (alpha-1,6-glucosidase) deficiency
What is deficient in a patient with diaphragm weakness, respiratory failure, and
cardiomegaly? What is the name of this disease? - Answer-Pompe disease (type II,
adult form): alpha-1,4-glucosidase deficiency
What is deficient in a patient with increased normal glycogen in the liver, elevated lactic
acid in the blood, hepatomegaly, hyperuricemia, and enlarged kidneys? What is the
name of this disease? - Answer-von Gierke's disease (type I): glucose-6-phosphatase
deficiency
In what part of the cell does the rate-limiting step of fatty acid synthesis synthesis
occur? - Answer-cytoplasm (acetyl CoA carboxylase)
Where in the cell does beta-oxidation occur? - Answer-mitochondrial matrix
In alcoholism, oxaloacetate is shuttled to malate due to excess of what? - Answer-
NADH
What is detected in the urine of a patient with ketoacidosis? - Answer-acetoacetate
, Between meals, what is the major source of glucose? - Answer-hepatic glycogenolysis
What are the sources of ATP in the first three days of starvation? - Answer-hepatic
glycogenosis
adipose release of free fatty acids
What triacylglycerol components can contribute to gluconeogenesis? - Answer-glycerol
and propionyl-CoA
In the first three days of starvation, what amino acids can be used in hepatic
gluconeogenesis? - Answer-lactate and alanine
What is the main source of fuel in muscles and the liver in the first three days of
starvation? - Answer-free fatty acids
What is the main source of energy in the brain and red blood cells after 3 days of
starvation? - Answer-ketone bodies
What determines survival time in starvation? - Answer-amount of adipose stores
What is the suspected diagnosis in an 2-month-old infant who displays failure to thrive,
steatorrhea, acanthocytosis, ataxia, and night blindness; intestinal biopsy shows lipid
accumulation within enterocytes? What is the inherited mutation? - Answer-
abetalipoproteinemia: mutation in the microsomal triglyceride transfer protein (MTP)
gene, leading to decreased production of B-48 and B-100
What enzyme is deficient in an infant who presents with lethargy, vomiting, and
hypotonia; labs reveal metabolic acidosis with large anion gap and elevated propionic
acid, ketosis, and hypoglycemia? - Answer-propionyl CoA carboxylase
What is the rate-limiting enzyme of glycolysis? - Answer-phosphofructokinase-1 (PFK-1)
What is the rate-limiting enzyme of gluconeogenesis? - Answer-fructose-1,6-
bisphosphatase
What is the rate-limiting enzyme of the TCA cycle? - Answer-isocitrate dehydrogenase
What is the rate-limiting enzyme of glycogen synthesis? - Answer-glycogen synthase
What is the rate-limiting enzyme of glycogenolysis? - Answer-glycogen phosphorylase
What is the rate-limiting enzyme of the HMP shunt? - Answer-glucose-6-phosphate
dehydrogenase (G6PD)
What is the rate-limiting enzyme of de novo pyrimidine synthesis? - Answer-carbamoyl
phosphate synthetase II (CPS II)
