USMLE Questions 2

Question 1

Blood tests show an increased amount of lactic acid at rest, with dramatic increases on exercise testing. Simultaneous measures of capillary oxygenation by a surface probe were normal. The abnormality most likely involves which of the following?

a) glycolysis in the lysosomes
b) glycolysis in the cytosol
c) respiratory chain in the mitochondria
d) glycogen breakdown in the mitochondria

Answer 1

respiratory chain in the mitochondria

Question 2

Her laboratory has ben working on metabolic inhibitor and that there is a high likelihood that she accidentally ingested one. Which of the following is the most likely culprit?

a) barbiturates
b) piericidin A
c) dimercaprol
d) dinitrophenol
e) cyanide

Answer 2

dinitrophenol

Question 3

Which of the following redox pairs, due to its high redox potential, is crucial for glutathione reduction?

a) fumarate/succinate
b) NADP+/NADPH or NAD/NADH
c) oxaloacetat/malate
d) pyruvate/lactate
e) 6-phosphogluconate/ribose-5-phosphate

Answer 3

NADP+/NADPH or NAD/NADH

Question 4

A patient positioned by which of the following compounds has the most highly reduced state of most of the respiratory chain carriers?

a) antimycin A
b) rotenone
c) carbon monoxide
d) puromycin
e) chloramphenicol

Answer 4

carbon monoxide

Question 5

Dehydrogenase can use which of the following compounds as an electron acceptor?

a) H2O
b) NAD+
c) O2
d) peroxide

Answer 5

NAD+

Question 6

The P450 cytochromes are members of which family of oxidoreductases?

a) catalase
b) hydroperoxidase
c) oxidase
d) oxygenase

Answer 6

oxygenase

Question 7

Which compound is a high-energy phosphate do not to ATP during glycolysis?

a) glucose-6-phosphate
b) glucose-1-phosphate
c) phophoenolpyruvate
d) phosphoglyceric acid
e) fructose-6-phosphate

Answer 7

phosphoenolpyruvate

Question 8

A muscular dystrophy is suspected, and a serum enzyme is measured that reflects the abundance of a high energy storage molecule in muscle. Which enzyme was measured?

a) adenosine triphosphate
b) creatine phosphokinase
c) glucose-1-phosphate dehydrogenase
d) pyrophosphatase

Answer 8

creatine phosphokinase

Question 9

Conversion of glucose to glucose-6-phosphase cannot take place in the cell unless the reaction is coupled to the hydrolysis of ATP. Which correctly explains why?

a) the reaction has a positive free energy
b) the reaction has a negative free energy
c) glucose-6-phosphate is unstable
d) there is on enzyme capable of catalyzing this reaction

Answer 9

the reaction has a positive free energy

Question 10

Which enzyme allows the high-energy phosphate of ADP to be used in the synthesis of ATP?

a) adenylyl kinase
b) ATPase
c) inorganic pyrophosphates
d) nucleoside diphosphate kinase

Answer 10

Adenylyl kinase

Question 11

Which reaction generates ATP during glycolysis?

a) glucose-6-phosphate to fructose-6-phosphate
b) glucose to glucose-6-phosphate
c) fructose-6-phosphate to fructose-1,6-diphosphate
d) phosphoenolpyruvate to pyruvate
e) pyruvate to lactate

Answer 11

phosphoenolpyruvate to pyruvate

Question 12

Which enzyme catalyzes high-energy phosphorylation of substrates during glycolysis?

a) pyruvate kinase
b) phosphoglycerate kinase
c) triose phosphate isomerase
d) aldolase
e) glyceraldehyde-3-phosphate dehydrogenase

Answer 12

glyceraldehyde-3-phosphate dehydrogenase

Question 13

Which enzyme product triacylglycerol breakdown and subsequent beta oxidation may undergo gluconeogenesis?

a) propionyl-CoA
b) acetyl-CoA
c) all ketone bodies
d) some amino acids
e) beta-hydroxybutyrate

Answer 13

propionyl-CoA

Question 14

Which regulates lipolysis in adipocytes?

a) activation of fatty acid synthesis mediated by cyclic AMP
b) activation of triglyceride lipase as a result of hormone-stimulated increases in cyclic AMP levels
c) glycerol phosphorylation to prevent futile esterification of fatty acids
d) activation of cyclic AMP production by insulin
e) hormone-sensitive lipoprotein lipase

Answer 14

activation of triglyceride lipase as a result of hormone-stimulated increases of cyclic AMP levels

Question 15

Which enzyme is directly inhibited by oligomycin?

a) pyruvate kinase
b) glucose-6-phosphate dehydrogenase
c) enolase
d) ATPase (ATP synthase)
e) phosphoenolpyruvate carboxylase

Answer 15

ATPase (ATP synthase)

Question 16

The yield from oxidation of triacylglycerise stores is which of the following?

a) 1 kcal/g
b) 2 kcal/g
c) 4 kcal/g
d) 9 kcal/g
e) 24 kcal/g

Answer 16

9 kcal/g

Question 17

Increased calcium concentration causes which of the following responses?

a) a dramatic rise in cyclic AMP levels
b) inactivation of glycogen phosphorylate
c) activation of phosphorylase kinase
d) activation of cyclic AMP phosphodiesterase
e) activation of protein phosphatase

Answer 17

Activation of phosphorylase kinase

Question 18

A liver biopsy reveals a very low level of acetyl-CoA carboxylase, but normal levels of the enzymes of glycolysis, gluconeogenesis, the citric acid cycle, and the pentose phosphate pathway. Which is the most likely cause of the infant’s respiratory problems?

a) low levels of phosphatidyl choline
b) biotin deficiency
c) ketoacidosis
d) high levels of citrate
e) glycogen depletion

Answer 18

Low levels of phosphatidyl choline

Question 19

How do oxidations involving NAD compare with those involving FAD?

a) NAD-linked oxidations generate 3 mol ATP per half mole of O2 consumed, whereas FAD-linked oxidations only generate 2 mol ATP per half mol of O2 consumed
b) FAD-linked oxidations generate 2 mol ATP per half mole of O2 consumed, whereas NAD-linked oxidations only generate 2 mol ATP per half mole of O2 consumed
c) Both oxidations generate 2 mol ATP per had mole of O2 consumed
d) Both oxidations generate 3 mol ATP per had mole of O2 consumed

Answer 19

NAD-linked oxidations generate 3 mol ATP per half mole of O2 consumed, whereas FAD-linked oxidations only generate 2 mol ATP per half mol of O2 consumed

Question 20

Which role is the coenzyme Q (ubiquinone) in the respiratory chain?

a) it links flavoproteins to cytochrome b, the cytochrome of lowest redox potential
b) it links NAD-dependent dehydrogenase to cytochrome b
c) it links each of the cytochromes in the respiratory chain to one another
d) it is the first step in the respiratory chain

Answer 20

it links flavoproteins to cytochrome b, the cytochrome of lowest redox potential

Question 21

In the resting state, what is the primary condition that limits the rate of respiration?

a) availability of ADP
b) availability of oxygen
c) availability of substrate
d) availability of both ADP and substrate

Answer 21

availability of ADP

Question 22

Oxidative phosphorylation couples generation of ATP with which of the following?

a) proton translocation
b) substrate level phosphorylation
c) electron flow through cytochromes
d) reduction of NADH
e) reduction of water

Answer 22

proton translocation

Question 23

Uncoupler 2,4-dinitrophenol used as weight reducing drug until side effects like fatigue and breathlessness precluded. How could the use of this drug result in weight loss?

a) 2,4-dinitrophenol is an allosteric activator of ATP synthase and this increases the rate of H+ translocation and oxidation of fats and other fuels
b) 2,4-dinitrophenol inhibits transport of pyruvate into the mitochondria. Fats are therefore metabolized to glycerol and subsequently to pyruvate, depleting fat stores
c) 2,4-dinitrophenol allows oxidation of fats in adipose tissue with our production of ATP. Fat oxidation can this proceed continuously and fat stores will be used up
d) 2,4-dinitrophenol causes ATP to be produced at a higher rate than normal, this causing weight loss

Answer 23

2,4-dinitrophenol allows oxidation of fats in adipose tissue without production of ATP. Fat oxidation can this proceed continuously and fat stores will be used up

Question 24

Which step is inhibited by carbon monoxide and cyanide?

a) oxidation step between cytochrome and coenzyme q
b) oxidation step involving direct reduction of oxygen
c) uncoupling of oxidation from phosphorylation
d) oxidation step between cytochromes c and b allow oxidative phosphorylation in the presence of oligomycin

Answer 24

oxidation step involving direct reduction of oxygen

Question 25

Important difference between respiratory chain inhibitors and uncouplers is which of the following?

a) the effect of respiratory chain inhibitors cannot be characterized spectroscopically, whereas that of uncouplers can
b) uncouplers do not inhibit electron transport, but respiratory chain inhibitors do
c) uncouplers are toxic substances, but respiratory chain inhibitors are not
d) respiratory chain inhibitors allow leakage of protons across the membrane but uncouplers do not

Answer 25

Uncouplers do not inhibits electron transport, but respiratory chain inhibitors do

Question 26

Cholera toxin causes massive and often fatal diarrhea by which of the following mechanisms?

a) inactivating Gi protein
b) irreversibly activating adenylate cyclase
c) locking Gs protein into an inactive form
d) rapidly hydrolyzing G protein GTP to GDP
e) preventing GTP from interacting with G protein

Answer 26

Irreversibly activating adenylate cyclase

Question 27

Why is the yield of ATP from the complete oxidation of glucose lower in muscle and brain than in kidney, liver, and heart?

a) difference shuttle mechanisms operate to transfer electrons from the cytosol to the mitochondria in the two sets of tissues
b) muscle and brain cells have a lower requirement for ATP
c) there are fewer mitochondria in muscle and brain cells
d) there are fewer ATP synthase in muscle and brain cells

Answer 27

Different shuttle mechanisms operate to transfer electrons from the cytosol to the mitochondria in the two sets of tissues

Question 28

The unavailability of carnitine to translocate fatty acids into mitochondria will cause which of the following?

a) inhibition of ATP synthase
b) depletion of NADH needed for oxidation
c) inhibition of cytochrome oxidase
d) inhibition of electron transfer from cytochromes to coenzyme Q
e) uncoupling of oxidation from phosphorylation

Answer 28

depletion of NADH needed for oxidation

Question 29

The problem of regenerating NAD+ from NADH for cytoplasmic processes by using mitochondria is solved in the most energy-efficient manner by which of the following?

a) reversing the direction of enzyme reactions like pyruvate dehydrogenase
b) locating certain cytochromes in the cytoplasm
c) shuttling of coupled reaction substrates like malate to aspartate
d) reversing the direction of glycolysis
e) direct oxidation of NADH by cytochrome P450

Answer 29

Shuttling of coupled reaction substrate like malate to aspartate

Question 30

Which tissue can metabolism glucose, fatty acids, and ketone bodies for ATP production?

a) liver
b) muscle
c) hepatocytes
d) brain
e) red blood cells

Answer 30

Muscle