USMLE Biochemistry Practice Questions

The correct answer is D. The patient’s symptoms
are consistent with those of seasonal allergies.
The patient has experienced asthma, eczema,
and allergies and most likely suffers from
atopy. Seasonal allergies are a result of histamine1-
receptor activation, which results in
pruritus, bronchoconstriction, and increased
nasal and bronchial mucus production. Seasonal
allergy symptoms can be treated with antihistamines
such as loratadine, which are histamine1-
antagonists.

The correct answer is B. This patient has
Pompe’s disease, a glycogen storage disorder.
Pompe’s disease is an autosomal recessive disease
that is characterized by a defi ciency or defect
in lysosomal α-1,4-glucosidase. This enzyme
is necessary for the dissolution of the
polymer linkages in glycogen. In its absence,
glycogen accumulates to toxic levels in both
the cytoplasm and lysosomes.

The correct answer is B. Glucokinase is found
in liver and pancreatic β cells. It catalyzes the
initial step of glycolysis, phosphorylation of
glucose to glucose-6-phosphate, which is catalyzed
by hexokinase in other tissues. Both enzymes
are found in the liver. Glucokinase has
a higher Michaelis-Menten constant (Km) and
a higher maximum reaction rate (Vmax) than
hexokinase; therefore, glucokinase has lower
affi nity for glucose and a higher capacity to
make glucose into glucose-6-phosphate. At low
glucose levels, hexokinase, with its higher affi
nity for glucose, processes glucose to glucose-
6-phosphate. At higher glucose levels, hexokinase
is overwhelmed (operating at its low Vmax)
and suffi cient substrate is available for glucokinase
to process the excess glucose even with its
low affi nity. Glucokinase thus helps to handle
large increases in glucose from the gut.

The correct answer is C. Type I dyslipidemia
is caused by a defi ciency of lipoprotein lipase.
This enzyme exists in capillary walls of adipose
and muscle tissue and cleaves triglycerides into
free fatty acids and glycerol. The enzyme is activated
by apolipoprotein C-II, which is found
on VLDL cholesterol and chylomicrons. Type
I dyslipidemia is characterized by an accumulation
of triglyceride-rich lipoproteins in the
plasma. It can also occur with an alteration in
apolipoprotein C-II.

The correct answer is D. This patient’s long
airplane fl ight placed her at an increased risk
of developing DVT. In addition, the patient
has a positive Homans’ sign (calf pain on dorsifl
exion), which is seen in some patients with a
DVT. Virchow’s triad refers to three factors that
predispose a patient to developing a venous
thrombosis: (1) local trauma to the vessel wall,
(2) stasis, and (3) hypercoagulability. Thromboxane
A2 stimulates platelet aggregation and
vasoconstriction and will be elevated at the site
of a clot.

The correct answer is B. This patient has adult
polycystic kidney disease, an autosomal dominant
condition characterized by massive bilateral
cysts in the kidneys (as shown in the image).
Cysts can also sometimes be found in the
liver. The renal cysts eventually lead to endstage
renal disease. In addition to the cysts, patients
are more prone to mitral valve prolapse
as well as berry aneurysms, which can rupture
and lead to strokes.

The correct answer is C. The 50S ribosomal
subunit is the larger of the two ribosomal macromolecules.
Its main function in translation is
the creation of a new peptide bond between
the incoming aminoacyl-tRNA and the growing
peptide chain. Essential for this purpose is
the 23S rRNA molecule, which holds the peptidyl
transferase enzyme activity. Linezolid is
an antibiotic that is active against gram-positive
bacteria including enterococci and staphylococci.
The linezolid mechanism of action is
inhibition of protein synthesis by binding to
the 23S rRNA and interacting with the initiation
complex. Linezolid resistance has been
reported in staphylococci and enterococci.
Since bacteria carry several copies of the 23S
rRNA gene, the number of rRNA genes mutated
seems to be a signifi cant determinant of
resistance.

The correct answer is C. The knockout mouse
model described exhibits a phenotype consistent
with impaired function of all of the hypothalamic
hormones with the exception of corticotropin-
releasing hormone (CRH). The hypothalamus
produces a number of important
hormones, including trophic hormone-releasing
hormone, that act on the anterior pituitary;
these include thyrotropin-releasing hormone
and gonadotropin-releasing hormone as well as
hormones to be stored and released in the posterior
pituitary, such as ADH and oxytocin. All of
the hypothalamic hormones exert their actions
on target cells via a phospholipase C (PLC) intracellular
signaling cascade with inositol trisphosphate
(IP3) as the second messenger, with
the exception of CRH, which acts via adenylate
cyclase-cAMP. Thus, the PLC-IP3 signaling system
is the most likely target of the gene knockout
described in the vignette. Important nonhypothalamic
hormones that also act via this
signaling system include angiotensin II and α1-
adrenergic agonists.

The correct answer is A. Hemoglobin carries
oxygen better when it is in the relaxed (R) form,
in which it has a higher affi nity for oxygen. As a
result, the oxygen dissociation curve shifts to the
left and decreased unloading of oxygen results.
Conversely, tightening hemoglobin decreases its
affi nity for oxygen. Hemoglobin in the T form is
stabilized by all the processes that result in increased
oxygen unloading. Binding of CO2 stabilizes
the T form, which decreases oxygen affi
nity.

The correct answer is D. Like steroid hormones,
this drug is binding to a cytosolic receptor
to form a hormone-receptor complex. The
complex is then transported to the nucleus,
where it acts on DNA to increase the transcription
of LDL cholesterol receptor mRNA, which
can be translated to LDL receptors.