USMLE USMLE Certifications USMLE Questions & Answers

• Question 221:

  A 47-year-old man with a history of sickle cell disease has had numerous hospitalizations requiring the placement of IV lines. The patient has poor peripheral venous access, and a catheter is placed in right subclavian vein. The patient subsequently develops right arm discomfort and swelling and a temperature of

  40.1 C with chills. Multiple blood cultures are taken, and gram-positive cocci are isolated. The organism is catalase positive and grows on mannitol salt agar, but does not turn the agar yellow; the colonies are gamma-hemolytic on a sheep blood agar plate. Which of the following organisms is the most likely cause of this patient's symptoms?

  A. Enterococcus faecalis

  B. Staphylococcus aureus

  C. Staphylococcus epidermidis

  D. Streptococcus agalactiae

  E. Streptococcus pyogenes

  Correct Answer: C

  Explanation:

  The patient has developed bacteremia; the description of the causative agent is consistent with a staphylococcal organism (catalase positive, gram-positive cocci that grow on mannitol salt agar). The organism is most likely Staphylococcus epidermidis as it was not able to ferment mannitol and was not hemolytic. Both of these characteristics tend to rule out Staphylococcus aureus. Two other tests that are commonly used are coagulase production and excretion of DNAs from colonies. S. aureus is positive in both tests, S. epidermidis is negative. Enterococcus faecalis might grow on the mannitol salt agar as it is relatively haloduric but these organisms are catalase negative. The enterococci are extremely variable in hemolytic ability so this characteristic is not useful in species identification. Both streptococcal organisms are catalase negative and beta-hemolytic on sheep blood agar plates. Also, neither would grow on the mannitol salt agar. Streptococcus pyogenes is sensitive to growth inhibition by bacitracin, whereas Streptococcus agalactiae (group B streptococci) is not.

• Question 222:

  While lying supine in bed eating, a child aspirates a peanut. Which of the following bronchopulmonary segments would this foreign object most likely enter?

  A. Apical segment of the left upper lobe

  B. Apical segment of the right upper lobe

  C. Medial segment of the right middle lobe

  D. Posterior basal segment of the left lower lobe

  E. Superior segment of the right lower lobe

  Correct Answer: E

  Explanation:

  Because the right main bronchus is wider and more vertical than the left, foreign objects are more likely to be aspirated into the right main bronchus. The superior segmental bronchus of the lower lobar bronchus is the only segmental bronchus that exits from the posterior wall of the lobar bronchi. Therefore, if a patient is supine at the time of aspiration, the object is most likely to enter the superior segmental bronchus of the lower lobe. None of the segmental bronchi of the left lung are likely to receive the object because the object is less likely to enter the left main bronchus. The apical segment of the right upper lobe is not likely to receive the foreign object because of the sharp angle that the upper lobar bronchus makes with the right main bronchus, and the sharp angle that the apical segmental bronchus makes with the lobar bronchus. The medial segmental bronchus of the right middle lobe arises from the anterior wall of the right middle lobar bronchus. Therefore, when the patient is supine, the effect of gravity will tend to prevent the object from entering this segmental bronchus.

• Question 223:

  A 54-year-old man presents to the emergency department with intense pain in his right eye. Examination reveals a red ring surrounding his iris and elevated intraocular pressure in the same eye. After obtaining a careful history with the aid of the man's wife, the emergency room physician concludes that this episode was triggered by which of the following agents?

  A. Amitriptyline

  B. Propranolol

  C. Cimetidine

  D. Diazepam

  E. Malathion

  Correct Answer: A

  Explanation:

  There are three facts that are necessary to answer this question: what disease the patient is suffering from, what pharmacological properties can trigger an attack, and what drug has these pharmacological properties. The patient described is suffering from an attack of acute or narrow angle glaucoma. These attacks can be precipitated by drugs with anticholinergic actions because muscarinic receptors on the pupillary constrictor muscle of the iris are blocked. This causes pupillary dilation, which further “narrows” the angle in the anterior chamber of the eye. Amitriptyline is a tricyclic antidepressant with significant anticholinergic side effects. Cimetidine is an H2 antagonist that reduces gastric acid release. Its trade name is Tagamet and it is now available over the counter. It has no significant anticholinergic side effects. Diazepam is a benzodiazepine. Its trade name is Valium and it has no significant anticholinergic side effects. Malathion is an organophosphorus cholinesterase inhibitor that is used as an insecticide. This agent would increase levels of acetylcholine, thereby widening the angle. Propranolol is a non-selective, beta-adrenergic antagonist. If anything, it would help to prevent an attack by blocking beta-receptors on the ciliary body, thereby diminishing aqueous humor production.

• Question 224:

  A 54-year-old man with extensive, severe atherosclerosis sustains a thrombotic occlusion of the celiac trunk. The organs that receive their blood supply from this artery continue to function normally. Anastomoses between which of the following pairs of arteries would explain this phenomenon?

  A. Left gastric artery and right gastric artery

  B. Left gastroepiploic artery and right gastroepiploic artery

  C. Proper hepatic artery and gastroduodenal artery

  D. Right colic artery and middle colic artery

  E. Superior pancreaticoduodenal artery and inferior pancreaticoduodenal artery

  Correct Answer: E

  Explanation:

  The superior pancreaticoduodenal artery is a branch of the gastroduodenal artery, which is a branch of the common hepatic artery, itself a branch of the celiac trunk. The inferior pancreaticoduodenal artery is a branch of the superior mesenteric artery. Occlusion of the celiac trunk would allow blood from the superior mesenteric artery to reach the branches of the celiac trunk via the connections between the superior and inferior pancreaticoduodenal arteries. Both the left and right gastric arteries receive their blood from the celiac trunk. The left gastric artery is a direct branch of the celiac trunk. The right gastric artery is usually a branch of the proper hepatic artery, which is a branch of the common hepatic artery (a branch of the celiac trunk).Both the left and right gastroepiploic arteries receive their blood supply from the celiac trunk. The left gastroepiploic artery is a branch of the splenic artery, which is a branch of the celiac trunk. The right gastroepiploic artery is a branch of the gastroduodenal artery, which is a branch of the common hepatic artery (a branch of the celiac trunk).The proper hepatic and gastroduodenal arteries are branches of the common hepatic artery, which is a branch of the celiac trunk. The right colic and middle colic arteries are both branches of the superior mesenteric artery.

• Question 225:

  In a normal individual, a tube with a transducer at its tip is swallowed and passed an unknown distance down the esophagus. Between swallows it records a pressure of 25 mm Hg. A small amount of water is swallowed. Within 2 seconds, the pressure falls to 5 mmHg, where it remains until returning to its resting pressure 6 seconds later. In a patient with achalasia, the transducer is advanced to the same location. Between swallows, it records a pressure of 30 mmHg. After swallowing, the pressure fails to decrease at all. In which of the following sites is the transducer most likely located?

  A. Esophageal body distal to the diaphragm

  B. Esophageal body proximal to the diaphragm

  C. Lower esophageal sphincter

  D. Pharynx

  E. Upper esophageal sphincter

  Correct Answer: C

  Explanation:

  Achalasia is an acquired esophageal motility disorder that is characterized by loss of enteric inhibitory neurons. The lower esophageal sphincter may exhibit increased tone in between swallows and fail to relax normally with a swallow. Peristalsis in the esophageal body is also abnormal. A swallow may not induce any peristalsis in the esophageal body or may produce simultaneous contractions along its entire length. The esophageal body distal to the diaphragm is relaxed in between swallows. The intraesophageal pressure at this point reflects the intra-abdominal pressure, which is slightly positive (5 mmHg). During inspiration, the pressure inside the distal esophagus rises along with the intra-abdominal pressure; during expiration this pressure falls. The pressure in the esophageal body proximal to the diaphragm reflects the intrathoracic pressure. It is slightly negative at the end of inspiration and slightly positive at the end of expiration. Since the mouth and pharynx are open to the atmosphere, in between swallows; the pressure within the pharynx is atmospheric (0 mmHg). The pressure rises abruptly to a maximum of 100 mmHg at the start of a swallow and returns to baseline within 0.5 seconds. At rest, the pressure in the upper esophageal sphincter can be as high as 60 mmHg. It is maintained by the normal elasticity of the sphincteric structures, as well as by active contraction of the cricopharyngeal muscle, which composes most of the sphincter. Shortly after the pharyngeal muscles contract during a swallow, the upper esophageal sphincter relaxes as the tonic neural input to the cricopharyngeal muscle (skeletal muscle) is inhibited as part of the swallowing program. Function of this sphincter is unaffected by achalasia.

• Question 226:

  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

  Correct Answer: C

  Explanation:

  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 astrocytoma.

• Question 227:

  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

  Correct Answer: C

  Explanation:

  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.

• Question 228:

  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

  Correct Answer: D

  Explanation:

  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.

• Question 229:

  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. 5α-reductase deficiency

  C. 17α-hydroxylase deficiency

  D. Sertoli-only syndrome

  E. Testicular dysgenesis

  Correct Answer: B

  Explanation:

  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 5α-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.

• Question 230:

  Which of the following is not considered a loop diuretic?

  A. Bumetanide (BUMEX)

  B. Furosemide (LASIX)

  C. Chlorothiazide (DIURIL)

  D. Ethacrynic Acid (EDECRIN)

  Correct Answer: C