Case files usmle step 1 ( sample )
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The bithermal caloric test, used to assess vestibular function, often causes vertigo and nausea when warm or cold water is introduced into the ear due to the convection currents set up in the endolymph of the inner ear. These currents can displace the cupula and inappropriately stimulate the vest
Case files usmle step 1 ( sample )
- 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. Case files – USMLE step 1 www.gims-org.com
- 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