Question 1
A 32-year-old construction worker arrives in the emergency department after an accident on the job. The tendon of the biceps brachii at the elbow has been severed by a laceration that extends 2 cm medially from the tendon. Which of the following structures is likely to have been injured by medial extension of the laceration?
A. Brachial artery
B. Ulnar nerve
C. Musculocutaneous nerve
D. Profunda brachii artery
E. Radial nerve
Question 2
A genotypic male (XY) is born with feminized external genitalia. The testes are retained within the abdominal cavity, and the internal reproductive tracts exhibit the normal male phenotype. Which of the following could account for this abnormal development?
A.Complete androgen resistance
B. 5a-reductase deficiency
C. 17a-hydroxylase deficiency
D. Sertoli-only syndrome
E. Testicular dysgenesis
Question 3
A 24-year-old AIDS patient develops chronic abdominal pain, low-grade fever, diarrhea, and malabsorption. Oocysts are demonstrated in the stool. Which of the following organisms is most likely to be the cause of the patient’s diarrhea?
A. Diphyllobothrium latum
B. Entamoeba histolytica
C. Giardia lamblia
D. Isospora belli
E. Microsporidia
Question 4
A 28-year-old man decides to donate a kidney to his brother, who is in chronic renal failure, after HLA typing suggests that he would be a suitable donor. He is admitted to the hospital, and his right kidney is removed and transplanted into his brother. Which of the following indices would be expected to be decreased in the donor after full recovery from the operation?
A. Creatinine clearance
B. Renal excretion of creatinine
C. Creatinine production
D. Daily excretion of sodium
E. Plasma creatinine concentration
Question 5
Bilateral ovarian masses are identified on pelvic examination of a 40-year-old woman. Ultrasound examination reveals multiloculated cystic masses involving both ovaries. The patient is treated with total abdominal hysterectomy with removal of both adnexa. Pathologic examination demonstrates papillary carcinoma producing serous fluid. Which of the following tumor markers would be most useful in monitoring for recurrence?
A. Alpha-fetoprotein
B. Bombesin
C. CA-125
D. PSA
E. S-100
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USMLE Question 1 Explanation: The answer is A. Brachial artery
The brachial artery is immediately medial to the tendon of the biceps brachii at the elbow. As the artery enters the forearm, it is covered by the bicipital aponeurosis, a broadening of the biceps tendon.
The musculocutaneous nerve does not cross the elbow. The musculocutaneous nerve gives off all of its muscular branches to muscles in the arm. The remainder of the nerve is then renamed the lateral cutaneous nerve of the forearm, which passes the elbow lateral to the tendon of the biceps.
The profunda brachii artery arises from the brachial artery in the proximal part of the arm. It accompanies the radial artery in the musculospiral groove and then divides into the radial collateral artery and middle collateral artery, which cross the elbow lateral to the tendon of the biceps.
The radial nerve lies within the musculospiral groove along the back of the humerus, then passes between the brachioradialis muscle and the brachialis muscle at the elbow, lateral to the tendon of the biceps.
The ulnar nerve crosses the elbow posterior to the medial epicondyle of the humerus. It then passes between the two heads of the flexor carpi ulnaris and courses through the forearm deep to this muscle.
USMLE Question 2 Explanation: The answer is B. 5a-reductase deficiency
In utero differentiation of the Wolffian ducts into the normal male phenotypic internal reproductive tract requires testosterone, but not dihydrotestosterone. On the other hand, differentiation of the indifferent external genital slit into the penis, prostate, and scrotum does require dihydrotestosterone. A congenital absence of 5a-reductase in these tissues will result in feminization. If left untreated, the affected individuals are generally phenotypic females until puberty, at which time increased amounts of testosterone result in virilization ("penis-at-twelve" syndrome). If discovered early, a male gender assignment can be supported with administration of dihydrotestosterone to increase penis size. If discovered after infancy, a female gender assignment can be supported with estrogen substitution therapy and prophylactic orchiectomy.
With complete androgen resistance, the external genitalia are feminized, but neither the male-type nor the female-type internal tracts develop. In the absence of the androgen receptor, the Wolffian ducts will degenerate. The Müllerian ducts will also degenerate because of the normal effect of testicular Müllerian regression factor.
With 17a-hydroxylase deficiency, the testes cannot synthesize testosterone, resulting in feminization of the external genitalia and degeneration of the Wolffian ducts. Normal secretion of Müllerian regression factor should also cause the degeneration of the Müllerian ducts. Because of the excessive secretion of deoxycorticosterone by the adrenal cortex, these individuals are usually hypertensive.
The Sertoli-only syndrome refers to the situation in which only the Sertoli cells of the seminiferous tubules are present (germinal cell aplasia). Spermatogenesis is absent in these individuals, who also show increased plasma levels of FSH because of decreased Sertoli cell secretion of inhibin. They may exhibit both male-type and female-type internal tracts because of the absence of Müllerian regression factor. The Leydig cells, however, have normal function and result in normal secretion of testosterone, so that both male-type internal tracts and external genitalia develop.
Testicular dysgenesis results in poor in utero development of the testes with concomitantly decreased secretion of testosterone and Müllerian regression factor. The Wolffian duct structures may degenerate, and the external genitalia may be feminized. Female-type internal tracts may develop because of the decreased secretion of Müllerian regression factor.
USMLE Question 3 Explanation: The answer is D. Isospora belli
All the organisms listed are protozoa. There are two intestinal protozoa specifically associated with AIDS that can cause transient diarrhea in immunocompetent individuals but can cause debilitating, and potentially life-threatening, chronic diarrhea in AIDS patients. These organisms are Isospora belli, treated with trimethoprim-sulfamethoxazole or other folate antagonists) and Cryptosporidium parvum (no treatment currently available).
Diphyllobothrium latum is the fish tapeworm and occasionally causes diarrhea.
Entamoeba histolytica and Giardia lamblia are both causes of diarrhea, but they are not specifically associated with AIDS.
Microsporidia are a protozoan cause of diarrhea but produce spores rather than oocysts.
USMLE Question 4 Explanation: The answer is A. Creatinine clearance
Because creatinine is freely filtered by the glomerulus, but not secreted or reabsorbed to a significant extent, the renal clearance of creatinine is approximately equal to the glomerular filtration rate (GFR). In fact, creatinine clearance is commonly used to assess renal function in the clinical setting. When a kidney is removed, the total glomerular filtration rate decreases because 50% of the nephrons have been removed, which causes the creatinine clearance to decrease. In turn, the plasma creatinine concentration increases until the rate of creatinine excretion by the kidneys is equal to the rate of creatinine production by the body. Recall that creatinine excretion = GFR x plasma creatinine concentration. Therefore, creatinine excretion is normal when GFR is decreased following removal of a kidney because the plasma concentration of creatinine is elevated.
Creatinine is a waste product of metabolism. Creatinine production is directly related to the muscle mass of an individual, but is independent of renal function.
The daily excretion of sodium is unaffected by the removal of a kidney. The amount of sodium excreted each day by the remaining kidney exactly matches the amount of sodium entering the body in the diet.
USMLE Question 5 Explanation: The answer is C. CA-125
The tumors are serous papillary cystadenocarcinomas of the ovaries. These tumors express CA-125 and are apparently derived from the surface epithelium of the ovaries.
Alpha-fetoprotein is not produced by this type of ovarian tumor but can be produced by testicular tumors and, less commonly, by ovarian tumors with a yolk sac tumor component.
Bombesin is a marker for neuroblastoma, small cell carcinoma, gastric carcinoma, and pancreatic carcinoma.
PSA is a marker for prostatic carcinoma.
S-100 is a marker for melanoma, neural tumors, and astrocytomas.
