MCT NBME Review
B correct answers (Marks' Medical Biochemistry Chapter 5)
A patient was diagnosed with one of the types of sphingolipidoses, which are congenital
diseases involving the ability to degrade sphingolipids. All sphingolipids have in
common which one of the following?
A. a glycerol backbone
B. ceramide
C. phosphorylcholine
D. N-acetylneuraminic acid (NANA)
B correct answers (Marks' Medical Biochemistry Chapter 6)
Protein kinases phosphorylate proteins only at certain hydroxyl groups on amino acid
side chains. Which of the following groups of amino acids all contain side-chain hydroxyl
groups?
A. aspartate, glutamate, serine
B. serine, threonine, tyrosine
C. lysine, arginine, proline
D. alanine, asparagine, serine
A correct answers (Marks' Medical Biochemistry Chapter 13, #5)
Autopsies of patients with Alzheimer disease show protein aggregates called
neurofibrillary tangles and neuritic plaques in various regions of the brain. These
plaques exhibit the characteristic staining of amyloid. Which of the following structural
features is the most likely characteristic of at least one protein in these plaques?
A. a high content of beta-pleated sheet structure
B. a high content of alpha-helical structure
C. a high content of random coils
D. disulfide bond cross-links between polypeptide chains
B correct answers (Marks' Medical Biochemistry Chapter 13, #5)
The key mechanistic failure in patients with xeroderma pigmentosum involves which of
the following?
A. Mutation in the primase gene
B. Inability to excise a section of the UV-damaged DNA
C. Mutation of one of the mismatch repair components
D. Inability to synthesize DNA across the damaged region
E. Loss of proofreading capacity
C correct answers (Marks' Medical Biochemistry Chapter 30, #3)
A newborn is diagnosed with neonatal jaundice. In this patient, the bilirubin produced
lacks which of the following carbohydrates?
A. glucose
,B. fructose
C. glucuronate
D. galactose
E. galactitol
D correct answers (Marks' Medical Biochemistry Chapter 31, #1)
A common intermediate in the conversion of glycerol and lactate to glucose is which of
the following?
A. pyruvate
B. oxaloacetate
C. malate
D. glucose-6-phosphate
E. phosphoenolpyruvate
A correct answers (Marks' Medical Biochemistry Chapter 31, #2)
A patient presented with a bacterial infection that produced an endotoxin that inhibits
phosphoenolpyruvate carboxykinase (PEPCK). In this patient, then, under these
conditions, glucose production from which of the following precursors would be
inhibited?
A. alanine
B. glycerol
C. even-chain fatty acids
D. phosphoenolpyruvate
E. galactose
C correct answers (Marks' Medical Biochemistry Chapter 31, #3)
Which of the following statements best describes glucagon?
A. It acts as an anabolic hormone.
B. It acts on skeletal muscle, liver, and adipose tissue.
C. It acts primarily on the liver and adipose tissue.
D. Its concentration in the blood increases after a high-carbohydrate meal.
E. Its concentration increases in the blood when insulin levels increase.
A correct answers (Marks' Medical Biochemistry Chapter 48, #4)
A patient presented with dysmorphia and cerebellar degeneration. Analysis of his blood
indicated elevated levels of phytanic acid and very long chain fatty acids, but no
elevation of palmitate. His symptoms are consistent with a defect in an enzyme involved
in which of the following?
A. alpha-oxidation
B. mitochondrial beta-oxidation
C. transport of enzymes into lysosomes
D. degradation of mucopolysaccharides
E. elongation of fatty acids
E correct answers (Marks' Medial Biochemistry Chapter 49, #1)
,Individuals who develop scurvy suffer from sore and bleeding gums and loss of teeth.
This is a result, in part, of the synthesis of a defective collagen molecule. The step that
is affected in collagen biosynthesis attributable to scurvy is which of the following?
A. The formation of disulfide bonds, which initiates tropocollagen formation
B. The formation of lysyl cross-links between collagen molecules
C. Secretion of tropocollagen into the extracellular matrix
D. The formation of collagen fibrils
E. The hydroxylation of proline residues, which stabilizes the collagen structure
B correct answers (Lippincott Review 6.1)
2,4-Dinitrophenol (DNP), an uncoupler of oxidative phosphorylation, was used as a
weight-loss agent in the 1930s. Reports of fatal overdoses led to its discontinuation in
1939. Which of the following would most likely be true concerning individuals taking 2,4-
DNP?
A. ATP levels in the mitochondria are greater than normal
B. Body temperature is elevated as a result of hypermetabolism
C. Cyanide has no effect on electron flow
D. The proton gradient across the inner mitochondrial membrane is greater than normal
E. The rate of electron transport is abnormally low
There is no transporter for NADH in the inner mitochondrial membrane. (However,
cytoplasmic NADH can be oxidizes to NAD+ by malate dehydrogenase as OAA is
reduced to malate. The malate is transported across the inner membrane to the matrix
where the mitochondrial isozyme of malate dehydrogenase oxidizes it to OAA as
mitochondrial NAD+ is reduced to NADH. This NADH can be oxidized by Complex I of
the ETC, generating 3 ATP through the coupled processes of oxidative
phosphorylation.) correct answers (Lippincott Review 6.3)
Explain why and how the malate-aspartate shuttle moves nicotinamide adenine
dinucleotide reducing equivalents from the cytosol to the mitochondrial matrix.
E
(vigorously contracting skeletal muscle shows an increase in the reduction of pyruvate
to lactate compared with resting muscle. The levels of reduced NADH increase and
exceed the oxidative capacity of the ETC. Consequently, the levels of AMP increase.
The concentration of F-2,6-BP is not a key regulatory factor in skeletal muscle. correct
answers (Lippincott Review 8.3)
Compared with the resting state, vigorously contracting skeletal muscle shows:
A. decreased AMP / ATP ratio
B. decreased levels of fructose 2,6-bisphosphate
C. decreased NADH / NAD+ ratio
D. increased oxygen availability
E. increased reduction of pyruvate to lactate
A
(The patient appears to have a thiamine-responsive PDHC deficiency. The pyruvate
decarboxylase (E1) component of the PDHC fails to bind TPP at low concentration but
, shows significant activity at a high concentration of the coenzyme. This mutation, which
affects the Km (Michaelis constant) of the enzyme for the coenzyme, is present in
some, but not all, cases of PDHC deficiency. Because the PDHC is an integral part of
carbohydrate metabolism, a diet low in carbohydrates would be expected to blunt the
effects of the enzyme deficiency. Aerobic glycolysis generates pyruvate, the substrate
of the PDHC. Decreased activity of the complex decreases production of acetyl CoA, a
substrate for citrate synthase. Because PDH kinase is allosterically inhibited by
pyruvate, it is inactive. correct answers (Lippincott Review 9.4)
A 1-month-old male shows neurologic problems and lactic acidosis. Enzyme assay for
pyruvate dehydrogenase complex (PDHC) activity on extracts of cultured skin
fibroblasts showed 5% of normal activity with a low concentration of thiamine
pyrophosphate (TPP) but 80% of normal activity when the assay contained a thousand-
fold higher concentration of TPP. Which one of the following symptoms concerning this
patient is correct?
A. Administration of thiamine is expected to reduce his serum lactate level and improve
his clinical symptoms.
B. A high-carbohydrate diet would be expected to be beneficial for this patient.
C. Citrate production from aerobic glycolysis is expected to be increased.
D. PDH kinase, a regulatory enzyme of the PDHC, is expected to be active.
D
(A low NAD+/NADH (oxidized to reduced nicotinamide adenine dinucleotide) ratio limits
the rates of the NAD+-requiring dehydrogenases. High availability of calcium and
substrate (acetyl coenzyme A) and a low ATP/ADP (adenosine tri- to diphosphate) ratio
stimulate the cycle.) correct answers (Lippincott Review 9.2)
Which one of the following conditions decreases the oxidation of acetyl coenzyme A by
the citric acid cycle?
A. a high availability of calcium
B. a high acetyl CoA / CoA ratio
C. a low ATP / ADP ratio
D. a low NAD+ / NADH ratio
C
(The girl is deficient in galactokinase and is unable to appropriately phosphorylate
galactose. Galactose accumulates in the blood (and urine). In the lens of the eye,
galactose is reduced by aldose reductase to galactitol, a sugar alcohol, which causes
osmotic effects that result in cataract formation. Deficiency of galactose 1-phosphate
uridylyltransferase also results in cataracts but is characterized by liver damage and
neurologic effects. Fructokinase deficiency is a benign condition. Aldolase B deficiency
is severe, with effects on several tissues. Cataracts are not typically seen.) correct
answers (Lippincott Review 12.4)
A 3-month-old girl is developing cataracts. Other than not having a social smile or being
able to track objects visually, all other aspects of the girl's examination are normal.
Tests on the baby's urine are positive for reducing sugar but negative for glucose.
Which enzyme is most likely deficient in this girl?
A. aldolase B
