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1. Global institute of medical sciences
CASE 1 - ANATOMY
A 2-year-old male patient develops progressive generalized weakness and muscle
atrophy.The impairment first began with the muscles of the hips, and then progressed to
the pelvic area,thigh, and shoulder muscles. The patient is diagnosed with Duchenne’s
muscular dystrophy,a congenital disorder where the protein dystrophin is deficient.
Which of the following describes the role of dystrophin in muscle tissue?
(A) anchors actin to the sarcolemma
(B) endows the myosin filaments with elastic
recoil properties
(C) extends from Z disk to Zdisk, forming a
supportive network
(D) inhibits the binding of myosin to actin
(E) protects desmin filaments from stressinduced
Damage
Explaination :
(A) Dystrophin anchors actin to the sarcolemma,reinforcing and stabilizing the latter
during muscle contraction. Titin is a large protein which associates with myosin
filaments and endows them with elastic recoil properties (choice B). Desmin filaments
form a supportive network extending from Z disk to Zdisk (choice C). Troponin I inhibits
the binding of myosin to actin (choice D). AlphaB-crystallin protects desmin filaments
from stress-induced damage (choice E).
Case files – USMLE step 1 www.gims-org.com

2. Global institute of medical sciences
CASE - 2 PHYSIOLOGY
For the bithermal caloric test, a measure of
vestibular sensitivity, warm or cold water is
introduced into the ear. This often causes vertigo
and nausea as a side effect as a result of
which of the following?
(A) convection currents in endolymph
(B) especially fast flow of endolymph
(C) temporary immobilization of cristae
ampullares
(D) temporary immobilization of otolith
organs
(E) temporary inhibition of cochlear hair
Cells
EXPLAINATION :
A) Water that is either warmer or cooler than body temperature when introduced into
the external auditory meatus sets up convection currents within the endolymph of the
inner ear.These currents may result in the movement of the cupula, the gelatinous
matrix, which sits atop the crista ampullaris and occludes the lumen of the semicircular
canal. Displacement of the cupula results in activation of the vestibular hair cells of the
ampullar crista. This inappropriate stimulation of the vestibular system conflicts with
sensory information from other sensor organs, including vestibular sensors on
the contralateral side, vision, and proprioception,and may in turn result in vertigo and
nausea. The existence of the many ducts within the inner ear might imply that the fluids
that fill them are flowing through the ear. However,neither endolymph nor perilymph
“flow” along their respective compartments in the normal or stimulated vestibular system
(choice B). Neither hair cells of cristae ampullares (choice C) nor of otholith organs
(choice D) nor of the cochlea (choice E) are temporarily immobilized or inhibited by
temperature changes.
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3. Global institute of medical sciences
CASE – 3 BIOCHEMISTRY
A deficiency of argininosuccinate synthetase (ASD) can be suspected in a 24–48-hour-
old neonate with elevated serum ammonia. To discriminate the fact that the
hyperammonemia is indeed due to ASD and not due to a deficiency in other urea cycle
enzymes, one can assay for the serum concentration of citrulline and urinary orotic acid
concentration. Which of the following would best describe the expected
findings?
(A) Both citrulline and orotate levels will be
low.
(B) Citrulline levels will be absent but orotate
will be elevated.
(C) Citrulline levels will be between
100–300 μM and orotate levels will be
elevated.
(D) Citrulline levels will be >1000 μM, orotate
will be near normal.
EXPLAINATION
D) A urea cycle disorder (UCD) is likely in a neonate that has elevated serum ammonia
appearing, not before, but between 24 and 48 hours after a normal term delivery. There
are three hallmark symptoms associated with UCDs.These are hyperammonemia,
encephalopathy, and respiratory alkalosis. Thus, elevated serum ammonia is not, in and
of itself, indicative of a specific defect in the urea cycle. An analysis of the levels of
various amino and organic acids in the plasma and urine is the primary key to
determining which defect led to the elevation in serum ammonia. Differential diagnosis
of neonatal hyperammonemia, as a consequence of a UCD can be accomplished by
measurement of plasma citrulline and urinary orotic acid levels.Shown in is the standard
differential diagnosis chart for determining which of four possible neonatal UCDs is the
cause of thehyperammonemia. First, the hyperammonemia appears in the absence of
any significant acidosis or ketosis. If analysis of serum citrullinedemonstrates that it is
>1000 μM it is that the clinical symptoms are due to a deficiency in ASD. In this
circumstance it is not necessary to assay for levels of urinary orotic acid but they would
be expected to be normal.Since citrulline levels are dramatically elevated in cases of
ASD, choices A, B, and C are not correct.
Case files – USMLE step 1 www.gims-org.com

4. Global institute of medical sciences
CASE 4 – MICROBIOLOGY
Astudent has a genetic defect and cannot produce the J chain which is important in the
structure of some immunoglobulin molecules.Which of the following will most likely
beobserved in this individual?
(A) a decrease in mature B cells
(B) a decrease in mature T lymphocytes
(C) a decrease in serum IgM
(D) an increase in IgA in the intestine
(E) an increase in serum IgM and decrease in IgE
EXPLAINATION :
(C) Deficiency in production of J chains for immunoglobulins could result in a decrease
in serum IgA and IgM levels. These antibodies are dimers and pentamers, respectively,
of the basic immunoglobulin molecule, and require a J chain to join the immunoglobulin
chains.There is no evidence which indicates that a deficiency in the production of J
chains of IgA or IgM will cause a decrease in mature B cells, or T lymphocytes, an
increase in serum IgM and decrease in IgE, or a decrease in IgA (choices A, B, D, and
E).
Case files – USMLE step 1 www.gims-org.com

5. Global institute of medical sciences
CASE – 5 PATHOLOGY
A 62-year-old woman has had atrial fibrillation since experiencing a myocardial
infarction (MI) 7 months prior. Two weeks ago she was hospitalized following a car
accident in which she suffered a compound fracture of her left femur and several severe
contusions. She now returns to the emergency room with right flank pain, hematuria,
and left-sided paralysis. These newly developing problems are most likely the
result of which of the following?
(A) air embolism from the compound
fracture
(B) bone marrow embolus from the fractured
femur
(C) fat embolism from the fractured femur
(D) systemic thromboemboli from the left
atrium
(E) venous thromboemboli from the deep
leg veins
EXPLAINATION :
D) Atrial fibrillation produces turbulence that is conducive to the formation of thrombi
which can then embolize throughout the systemic circulation. In this patient the right
flank pain and hematuria and left-sided paralysis suggest that thromboemboli traveled
to the right kidney and the brain, respectively.Embolism of air (choice A) has variable
effects,although small amounts are typically inconsequential.Larger volumes (~ >100
mL) can obstruct arteries and lead to ischemia and necrosis; an air embolism that
lodges in the right heart may lead to sudden death.However, air embolism is rare and is
not the most likely event in this patient. Bone marrow embolism (choice B) can occur
following a broken bone or cardiac resuscitation, but typically will have no clinical
consequences.However, fat embolism (choice C) from abroken long bone or
traumatized areas of adipose tissue can, when severe, produce clinical manifestations.
These follow the trauma by about 1–3 days and usually include dyspnea,skin rash, and
acute neurologic changes. In a few cases this has been fatal. Venous thromboemboli
from deep leg veins (choice E) could not get past the lungs (with the unusual exception
of paradoxical embolism in a person with a septal defect) so could not reach thekidneys
or brain to cause the effects reported for this patient.
Case files – USMLE step 1 www.gims-org.com

6. Global institute of medical sciences
Case files – USMLE step 1 www.gims-org.com