A previously healthy 71-year-old man comes to the emergency department because of a 12-hour history of fever and cough. Current medications include ramelteon nightly. His temperature is 39.6°C (103.2°F), pulse is 100/min, respirations are 20/min, and blood pressure is 100/60 mm Hg. Crackles are heard over the right lower lung with dullness to percussion. Laboratory studies show:

A chest x-ray shows a patchy infiltrate in the right lower lobe. A Gram stain of sputum shows numerous segmented neutrophils containing gram-positive cocci. A photomicrograph of a Wright-stained peripheral blood smear is shown. Which of the following is the most likely explanation for these findings?

A 16-year-old girl is brought to the emergency department because of a 2-hour history of rash and burning pain on her shoulders and arms. She had been hiking outdoors on a sunny day for several hours before the symptoms began. Her only medication is oral tetracycline for facial acne. Her temperature is 37°C (98.6°F). Physical examination shows diffuse erythema over the shoulders and upper extremities, and bullous lesions over the shoulders. Which of the following is the most likely diagnosis?

A 16-year-old girl is brought to the physician because of excessive dieting, constant studying, and social withdrawal during the past 3 weeks. She is 168 cm (5 ft 6 in) tall and weighs 48 kg (106 lb); BMI is 17 kg/m2. Her blood pressure is 95/66 mm Hg. Physical examination shows dry skin and fine hair growth. Laboratory studies show:

This patient has most likely been abusing which of the following substances?

A 35-year-old woman comes to the physician because of severe nonradiating low back pain for 2 weeks. She recently began a vigorous exercise program. Her back pain has been partially relieved by bed rest and aspirin. She has ulcerative colitis that has been treated with corticosteroid therapy for 5 years and is currently in remission. Physical examination shows mild localized tenderness over several lumbar vertebrae. A lateral x-ray of the lumbar spine is shown. Which of the following best explains these radiographic findings?

A 37-year-old woman is found to have a 1-cm thyroid nodule on routine physical examination. There is no adenopathy. Fine-needle aspiration of the nodule confirms papillary thyroid carcinoma. She undergoes thyroidectomy and postoperative radioiodine (131l) ablation. When the radioiodine is administered, there is a large concentration of homogenous iodine uptake in the superior mediastinum. Which of the following best explains this finding?

Virus isolates are obtained from the blood of a man infected with HIV before and after 1 year of treatment with a drug regimen that includes zidovudine (AZT). Sensitivity testing for zidovudine shows:

The viral gene most likely to have mutated during the year to cause this change is the one encoding which of the following proteins?

An 18-year-old man has had temperatures to 38.3°C (101°F), a sore throat, and cervical lymph node enlargement for 8 days. A latex agglutination test result for Epstein-Barr virus antibody is positive. Atypical cells with abundant lacey cytoplasm in the peripheral blood smear are most likely derived from which of the following types of cells?

Within the Ashkenazi Jewish population, the frequency of Tay-Sachs disease is approximately 1 in 3600. Assuming that this population is in Hardy-Weinberg equilibrium, which of the following is the expected frequency of heterozygous carriers within this population?

A 53-year-old man is brought to the emergency department because of progressive fever, cough, fatigue, and shortness of breath during the past 3 days. His temperature is 39.2°C (102.6°F), and respirations are 21/min. Physical examination shows diaphoresis. There is increased tactile fremitus, dullness to percussion, and bronchovesicular breath sounds over the left lower hemithorax posteriorly. A chest x-ray is shown. Arterial blood gas analysis on oxygen via nasal cannula (2 L/min) shows:

Which of the following is the primary cause of the hypoxemia in this patient?

A 1-week-old male newborn has the lesion shown. Which of the following is the most likely diagnosis?

A screening program was instituted for detection of depressive symptoms among college students. At the first examination, evidence of depression was found in 250 of 1250 subjects. During the following year, an additional 100 students developed depressive symptoms. Which of the following is the annual incidence of depression in this population of students?

Restriction of valine in the diet is most likely to be effective in treatment of patients with which of the following?

A 72-year-old man with chronic obstructive pulmonary disease comes to the physician for a follow-up examination. Current medications include albuterol and ipratropium. He smokes 1 pack of cigarettes daily. He does not drink alcohol. His respirations are 24/min. Pulse oximetry on room air shows an oxygen saturation of 87%. Breath sounds are decreased bilaterally. His baseline FEV1 is 0.88 (38% of predicted). Which of the following hematologic abnormalities is most likely in this patient?

A 23-year-old woman comes to the physician because of a 3-week history of fatigue and muscle pain shortly after she begins exercising; her symptoms decrease as exercise proceeds. Physical examination shows no abnormalities. Her fasting serum glucose concentration is 90 mg/dL. Following exercise of the left forearm, her serum lactic acid concentration does not increase in blood obtained from the left antecubital vein; however, ammonia concentration does increase. A muscle biopsy specimen is positive for periodic acid-Schiff staining. This patient most likely has a deficiency of which of the following muscle enzymes?

A 35-year-old woman, gravida 1, para 1, comes to the physician because of a 2-month history of generalized weakness and fatigability that are significantly exacerbated by exercise. The symptoms began during her pregnancy. Physical examination shows diplopia. There is reduction in arm muscle strength with repetitive movement, but preservation of deep tendon reflexes. A therapeutic trial of oral pyridostigmine provides symptomatic relief. Serum studies will most likely show antibodies to which of the following proteins?

A 10-year-old boy who has had type 1 diabetes mellitus for 1 year is receiving insulin. One hour after his morning dose of insulin, he becomes tremulous and diaphoretic and has tachycardia. Several hours later, his symptoms resolve. His blood glucose concentration is now increased. Which of the following is the most likely cause of this patient’s hyperglycemia?

A 74-year-old woman is admitted to the hospital to undergo a cholecystectomy. During the operation, she becomes acutely hypotensive. The hypotension persists for 5 minutes despite administration of appropriate pressor agents. Which of the following morphologic lesions is most likely to occur in this patient as a result of this incident?

During an experimental study, a glutamate antagonist is administered to 100 patients with a history of acute cerebral infarction to target the N-methyl-D-aspartate receptor. Which of the following is the intended result of this therapy?

A 47-year-old man comes to the physician because of a 2-week history of a persistent, nonproductive cough. He has a 3-year history of hypertension treated with a thiazide diuretic. A second drug was added to his medication regimen 2 weeks ago. Vital signs are within normal limits. Pulmonary examination shows no abnormalities. Which of the following drugs was most likely added to this patient’s medication regimen?

A 70-year-old man comes to the emergency department 2 hours after the sudden onset of severe left-sided low back pain that radiates to his abdomen. He has a 25-year history of hypertension and type 2 diabetes mellitus. He is in severe distress. His pulse is 110/min, and blood pressure is 80/60 mm Hg. Physical examination shows diaphoresis and a soft, nontender abdomen; there is no tenderness of the back or spine. Neurologic examination shows no abnormalities. Urinalysis shows no blood. Which of the following is the most likely diagnosis?

A 25-year-old woman comes to the physician because of a 7-year history of 5- to 10-minute episodes of increased heart rate, sweating, trembling, dizziness, derealization, and intense fear that occur almost daily. She feels well between attacks except for apprehension about when the next attack will occur. Physical examination and laboratory studies show no abnormalities. Which of the following is the most likely diagnosis?

A previously healthy 26-year-old woman comes to the physician because of a 1-day history of severe rectal pain with no bleeding. She takes no medications and does not smoke cigarettes or drink alcohol. She has been sexually active, and she and her partner use condoms inconsistently. A photograph of the rectal area is shown. Which of the following is the most likely diagnosis?

A 53-year-old man comes to the physician to learn the results of a series of tests to evaluate his severe abdominal pain. The testing occurred over a 3-week period and the recent results indicate pancreatic cancer. When the physician reviews the results with the patient and discusses the diagnosis, the patient becomes angry and asks, “Why did it take you so long to figure this out?” Which of the following is the most appropriate response by the physician?

A 35-year-old man comes to a new physician because of a 3-day history of abdominal pain and daily loose stools. He has a 2-year history of recurrent epigastric pain and a 4-month history of peptic ulcer disease treated with acid suppression medications. He takes no other medications. He was told on three separate occasions that he had ulcers in his duodenum and jejunum but no infection. His blood pressure is 120/78 mm Hg. The abdomen is soft. There is tenderness to palpation of the epigastric area but no rebound tenderness. Which of the following is the most likely cause of this patient’s condition?

A 60-year-old man comes to the physician for an examination prior to employment. Physical examination shows no abnormalities. Laboratory studies show:

Which of the following is most predictive of a clonal lymphoid proliferation in this patient?

A 10-year-old boy with cerebral palsy begins benzodiazepine (BZ) therapy with diazepam. The mechanism of action of this drug occurs in the central nervous system where it interacts with the y-aminobutyric acid (GABA) receptor-ion channel macromolecular complex. Which of the following illustrations shown best represents the manner in which diazepam interacts with the macromolecular complex and the nature of the ion flux that subsequently occurs in this patient?

A 25-year-old man comes to the physician 10 days after injuring his right elbow while trying to lift an 80-lb dumbbell at the gym. When he attempted to lift the weight from the rack, he felt a popping sensation and pain in his right elbow. He uses anabolic steroids. His pulse is 78/min, and blood pressure is 150/95 mm Hg. Examination of the right upper extremity shows swelling of the antecubital fossa and ecchymoses from the antecubital fossa to the mid forearm on the anterior side. There is tenderness to palpation over the proximal aspect of the anterior forearm. He is able to flex the elbow actively, but he is unable to flex the elbow or supinate the forearm against resistance. This patient has most likely injured a muscle-tendon unit that is supplied by which of the following nerves?

A 40-year-old woman comes to the physician because of a 3-month history of continuous watery diarrhea that has gradually increased in severity; she has had an 11-kg (24-lb) weight loss during this period. Physical examination shows skin tenting and sunken eyes, and orthostatic hypotension. Tests of the stool are negative for ova, parasites, and Clostridium difficile. A CT scan of the abdomen shows a mass in the pancreas. Treatment with standard antidiarrheal medications is ineffective. Administration of a somatostatin analogue, octreotide, results in improvement of the diarrhea. This patient most likely has a tumor secreting which of the following agents?

A 30-year-old primigravid woman develops severe uterine bleeding in the hospital after vaginal delivery of a 3827-g (8-lb 7-oz) male newborn at term. Pregnancy was  uncomplicated. Delivery lasted 7 hours and was assisted with the use of forceps; delivery of the placenta followed. External uterine massage does not result in decrease of the postpartum hemorrhage. A medication is administered by continuous intravenous infusion to improve uterine contraction. This patient’s bleeding is most likely to be decreased via which of the following mechanisms of action?

A 45-year-old man comes to the physician for evaluation of worsening heartburn, which has been present for the past 15 months. The pain has become more frequent and severe in the past month and is no longer relieved by antacids. He also has occasional trouble swallowing. Endoscopic examination shows erythema of the lower third of the esophagus. Microscopic biopsies of this area show basal zone hyperplasia and an eosinophilic infiltrate of the squamous mucosa. Which of the following is the most likely diagnosis?

A 42-year-old man develops supraventricular tachycardia 15 minutes after undergoing an abdominal operation. An intravenous infusion with an ultrashort-acting (3-adrenoreceptor blocking agent is started. This drug is most likely which of the following?

Protection against paralytic poliomyelitis can be achieved either by oral administration of live attenuated poliovirus (Sabin) vaccine or by parenteral administration of killed poliovirus (Salk) vaccine. The common features of these two vaccines, which accounts for their efficacy, is their ability to induce which of the following poliovirus-specific immune responses?

An experimental animal has a blood flow of 4 mL/sec in a small arterial segment of the mesenteric circulation. Which of the following best describes the blood flow (in mL/sec) if the diameter of the vessel is decreased by one half with no change in the perfusion pressure gradient?

A 29-year-old woman is brought to the emergency department 1 hour after being injured in a motor vehicle collision. Abdominal examination shows tenderness with rebound. Vital signs are stable. A CT scan of the abdomen shows no abnormalities except for a small amount of intraperitoneal fluid. After 6 hours of observation, an exploratory laparotomy is done because of continuing severe abdominal pain. No abnormalities are seen until the surgeon gently slides his hand behind the liver; there is a sudden massive hemorrhage from the region of the hand, and the patient’s blood pressure becomes undetectable within 1 minute. Which of the following underlying lesions is most likely associated with the hemorrhage?

A 56-year-old woman comes to the emergency department because of a 1 -month history of a painful, enlarging mass on the right side of her jaw. She has not noticed any drainage from the area. She has had no fever or other systemic symptoms. She appears thin. Her vital signs are within normal limits. Physical examination shows a tender, 3-cm, firm mass on the right mandible with mild overlying erythema and warmth but no exudate. Oral examination shows multiple dental caries. There is shotty anterior right cervical lymphadenopathy. A CT scan of the head shows a heterogeneous, 3-cm, soft-tissue mass with associated bony changes indicative of osteomyelitis. A photomicrograph of a specimen obtained via needle aspiration of the mass is shown; aerobic cultures grow no organisms. Which of the following is the most likely causal organism?

A full-term female newborn is born with a symmetrically enlarged neck. The swelling is composed of a mass of endothelial cell-lined, cystically dilated spaces filled with lymph. The intervening stroma is scanty, and there are occasional lymphoid aggregates. Which of the following disorders is most likely in this newborn?

A 65-year-old man receives a liver transplant for primary biliary cirrhosis. To prevent organ rejection, immunosuppressive therapy that inhibits T-lymphocyte activation with cyclosporine is planned. The activity of which of the following molecules involved in T-lymphocyte activation is most likely to be inhibited by this treatment?

A 30-year-old woman has diarrhea and crampy abdominal pain. Which of the following historical features most strongly suggests that the cause is inflammatory bowel disease rather than irritable bowel syndrome?

A 23-year-old woman comes to the physician for prenatal care. On examination, she appears euthyroid and the thyroid gland is not palpable. Uterine size is consistent with dates, and fetal pulse is normal. Serum thyroid tests show:

Changes in which of the following hormones during pregnancy is primarily responsible for the thyroid test results?

A 25-year-old woman comes to the physician because of palpitations and insomnia for 5 months; she also has had a 5-kg (11 -lb) weight loss during this period. Her pulse is 125/min. Physical examination shows exophthalmos, thyromegaly, and hand tremors. Serum studies show a thyroid-stimulating hormone concentration of less than 0.1 pU/mL, thyroxine (T4) concentration of 18 pg/dL, and triiodothyronine (T3) concentration of 250 ng/dL. The signs and symptoms in this patient are most likely caused by hormones acting on which of the following receptors?

A 27-year-old woman with a 15-year history of recurrent urinary tract infections develops hypertension, proteinuria, and renal failure. Her serum creatinine concentration is 2.6 mg/dL. X-rays show irregular scarring of the kidneys and blunting of the renal calyces, most prominent in the upper and lower poles. Which of the following is the most likely cause of the renal abnormalities?

A 52-year-old man with acute myelogenous leukemia comes to the physician because of a 3-day history of fever and a 2-week history of nonproductive cough. He received  ablative chemotherapy and received a bone marrow transplant 2 months ago. He has had neutropenia since the transplant. His temperature is 38.8°C (101.8°F), pulse is 96/min, respirations are 20/min, and blood pressure is 100/65 mm Hg. Crackles are heard over the right lung base. His serum 1,3-β-D-glucan concentrations are increased. A chest x-ray shows a cavitary lesion in the right lower lobe. This patient’s serum study result most likely indicates the presence of which of the following?

A 22-year-old man is brought to the emergency department 15 minutes after sustaining a gunshot wound to the abdomen. His pulse is 110/min, respirations are 20/min, and blood pressure is 100/60 mm Hg. Abdominal examination shows an entrance wound in the left upper quadrant and no exit wound. A CT scan of the abdomen is shown. The injured organ is removed. The bullet is most likely lodged in which of the following organs?

A 60-year-old man comes to the physician for a routine health maintenance examination. Physical examination shows no abnormalities. Urinalysis shows:

Microscopic examination of the urine shows atypical cells. A CT scan of the abdomen discloses a lesion of the right kidney as shown in the photograph. Which of the following compounds is the most significant predisposing risk factor for this patient’s condition?

A 24-year-old man comes to the physician because his wife has been unable to conceive for the past 2 years. Previous evaluation of his wife showed no abnormalities. He has no history of major medical illness. He does not smoke cigarettes. Physical examination shows a left testicle that is smaller than the right and an enlarged left spermatic cord that increases in size with Valsalva maneuver. Laboratory studies show oligospermia. Which of the following is the most likely diagnosis?

A previously healthy 55-year-old man comes to the physician because of a 2-day history of pain with urination, urinary frequency, low back pain, and nausea. His temperature is 38.5°C (101.3°F), pulse is 88/min, respirations are 20/min, and blood pressure is 144/88 mm Hg. Physical examination shows acute low back tenderness. Urinalysis shows 3 protein; numerous WBCs, epithelial cells, and bacteria; and a few RBCs. Treatment with a quinolone antibiotic is initiated, but his symptoms do not resolve. Urine culture grows Escherichia coli resistant to ciprofloxacin. The most likely cause of this resistance is a mutation in which of the following elements?

A 3-year-old boy has progressive coarsening of his facial features, hirsutism, hepatomegaly, and developmental delay. X-rays of the skeletal system show dysostosis multiplex. Which of the following is primarily involved in this condition?

A study is designed to evaluate the relationship between ambient noise and hearing loss in an automobile manufacturing plant. Different locations in the manufacturing plant have strikingly different levels of ambient noise. Each employee is given a test for hearing acuity, and then the ambient noise level is measured at his or her workstation. Which of the following best describes this study design?

A 65-year-old woman with cirrhosis comes to the physician because of progressive shortness of breath during the past 3 months. Her pulse is 94/min, respirations are 20/min, and blood pressure is 100/60 mm Hg. Physical examination shows ascites and marked edema of the lower extremities up to the hips. A chest x-ray shows bilateral pleural effusions. Echocardiography shows a left ventricular ejection fraction of 55%. Which of the following is the most likely cause of the increased intra-alveolar fluid in this patient?

A 6-year-old boy who recently emigrated from Russia is brought to the physician by his parents because of unstable gait and incoordination for 2 weeks. He has had frequent pale, bulky stools for 4 years and two episodes of bacterial pneumonia and chronic cough since the age of 1 year. He is below the 3rd percentile for height and weight. Increased rhonchi are heard over both lung fields. Neurologic examination shows ataxia, absence of deep tendon reflexes, and loss of proprioception. Stool analysis shows an increased fat concentration. Which of the following vitamins is most likely deficient in this patient?