USMLE Biochemistry 2

Question 1

A child presents with megaloblastic anemia and homocystinuria but normal methylmalonic acid levels. Which vitamin deficiency is most likely?

Accepted Answer

Folate

Answer

Folate deficiency causes megaloblastic anemia and elevated homocysteine but does not cause methylmalonic aciduria, distinguishing it from B12 deficiency.

Question 2

Which glycogen storage disease presents with hypoglycemia, lactic acidosis, and hepatomegaly due to glucose-6-phosphatase deficiency?

Accepted Answer

Von Gierke disease (Type I)

Answer

Von Gierke disease (Type Ia) results from glucose-6-phosphatase deficiency, preventing glucose release from the liver and causing hypoglycemia with lactic acidosis.

Question 3

In the citric acid cycle, which reaction is catalyzed by isocitrate dehydrogenase?

Accepted Answer

Isocitrate → alpha-ketoglutarate + CO2

Answer

Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate, producing CO2 and NADH.

Question 4

Alkaptonuria is caused by a deficiency of which enzyme?

Accepted Answer

Homogentisate oxidase

Answer

Alkaptonuria results from homogentisate oxidase deficiency, causing accumulation of homogentisic acid, which causes dark urine and ochronosis.

Question 5

Which lipoprotein is primarily responsible for reverse cholesterol transport?

Accepted Answer

HDL

Answer

HDL mediates reverse cholesterol transport by collecting cholesterol from peripheral tissues and delivering it to the liver for excretion.

USMLE Practice Test

USMLE Practice Test

USMLE Biochemistry 2