An investigator obtains a serum sample from an individual previously immunized with a vaccine that elicits immunity to Clostridium tetani. Administration of this serum to a guinea pig prior to challenge with a potentially lethal dose of C. tetani is most likely to mediate protection to the challenge dose through binding of antibody to which of the following products of the bacterial cells?

A 66-year-old woman comes to the physician for a follow-up examination 3 months after she underwent a total colectomy for ulcerative colitis. She subsequently required an ileostomy. She feels generally well and has resumed a nearly normal diet. Her temperature is 37°C (98.6°F), pulse is 92/min, and blood pressure is 100/60 mm Hg. Physical examination shows mildly decreased skin turgor and a well-healed ileostomy site. Laboratory studies show:

Which of the following best describes this patient’s acid-base disturbance?

A 56-year-old man dies 1 month after the onset of difficulty with short-term memory and anxiety. He underwent surgical excision of small cell carcinoma 1 year ago. Degenerative changes are most likely to be concentrated at which of the following labeled sites on the normal MRI of the head?

The illustration shows the arachidonic cascade that is activated when skin is exposed to poison ivy. Within this cascade, which of the following is the major mechanism of analogs of corticosteroids?

A 15-year-old girl is brought to the physician because of a 3-day history of fever, sore throat, and malaise. Her temperature is 39.2°C (102.6°F). Physical examination shows diffuse pharyngeal erythema, moderately enlarged tonsils, and tender anterior and posterior cervical lymphadenopathy. A complete blood count shows:

Incubation of this patient’s serum with sheep erythrocytes results in agglutination. The atypical lymphocytes in this patient are most likely which of the following cell types?

A 68-year-old woman comes to the physician because of a 1-year history of severe abdominal pain after meals; she also has had an unintentional 9-kg (20-lb) weight loss during this period. The pain is relieved when she decreases the amount of food that she eats. She has a history of atherosclerosis and underwent triple coronary artery bypass grafting 2 years ago. Physical examination shows a soft, nontender abdomen and an abdominal bruit. Pedal pulses are diminished. The most likely cause of this patient’s symptoms is stenosis of which of the following arteries?

An 1814-g (4-lb) female newborn is delivered in the hospital at 24 weeks’ gestation to a 25-year-old primigravid woman via a spontaneous vaginal delivery. Examination of the newborn shows no peripheral cyanosis. There is a grade 3/6, continuous murmur over the left midclavicular, second intercostal space. At the age of 2 days, the newborn develops respiratory distress. Echocardiography shows a failure of a vascular structure to close. The embryologic origin of this anomaly is most likely which of the following aortic arches?

A study is conducted to assess 32 patients in a community of 1000 who have developed drug-resistant tuberculosis during a 1-year period. These patients are removed from the community for treatment. Assuming that the risk for infection and susceptibility to the disease is constant, which of the following best represents the number of individuals most likely to develop subsequent drug-resistant tuberculosis during the next year?

A homeless 45-year-old man comes to the clinic because of localized back pain for the past 3 months. He has not seen a health care provider in more than 20 years. Physical examination shows tenderness over the lower thoracic spine. There are no obvious neurologic deficits. X-rays of the spine show narrowing of the disc space between T10 and T11 and destruction of the adjacent T10 vertebral body. Which of the following is the most likely cause of these findings?

A 22-year-old man is admitted to the hospital for treatment of a ruptured appendix. His temperature is 38.9°C (102°F). Abdominal examination shows guarding, rebound tenderness, and extreme tenderness of the right lower quadrant. He undergoes exploratory laparotomy, and an abscess containing foul-smelling pus is found. A Gram stain of exudate shows numerous segmented neutrophils, gram-positive cocci, gram-positive rods, and gram-negative rods. Aerobic culture of the exudate at 37°C on blood and MacConkey agar plates yields only enterococci. Which of the following bacteria is most likely to be among the gram-negative rods?

A 32-year-old woman comes to the physician because she is concerned about fertility. Menses occur at regular 28-day intervals. Physical examination shows no abnormalities. An endometrial biopsy specimen is obtained from the uterine wall. Histologic sections show a well-developed stratum basale, intact spiral arteries, abundant endometrial stroma, and long, straight uterine glands with no secretions in their lumens. Based on this description, which of the following labeled options best corresponds to the point in the patient’s menstrual cycle at which the biopsy was done?

A 40-year-old man has moderate edema. A 24-hour urine collection contains 15 g of protein (N<150 mg/24 h). Which of the following is the most likely mechanism of the edema?

A 14-year-old girl is brought to the physician by her mother because of episodes of increasingly severe facial blemishes during the past 6 months. She says that she does not eat high-fat foods or use makeup. Use of an astringent soap has not resolved her symptoms. Menarche occurred at the age of 12 years. A photograph of her face is shown.
Which of the following best describes the pathologic mechanism of this patient’s condition?

A 75-year-old man with alcoholism is brought to the emergency department 30 minutes after he began vomiting blood. He has a history of portal hypertension. Abdominal examination shows spider angiomata. Which of the following is the most likely cause of this patient’s hematemesis?

A 22-year-old woman comes to the physician because of a 6-month history of frequent headaches and fatigue. She tells the physician that she often overeats when she is anxious and has been making herself vomit and using laxatives for the past 2 years. She is 168 cm (5 ft 6 in) tall and weighs 59 kg (130 lb); BMI is 21 kg/m2. Physical examination will most likely show which of the following conditions?

A 75-year-old woman with non-small cell carcinoma of the lung is brought to the physician because of a 2-day history of lethargy and cognitive impairment. One month ago, she completed a 1-year regimen of chemotherapy and cranial irradiation. One week ago, she received the diagnosis of major depressive disorder and began therapy with a selective serotonin reuptake inhibitor. She speaks with a blunted affect. Physical examination shows no abnormalities. Her Mini-Mental State Examination score is 20/30. Serum studies show a sodium concentration of 122 mEq/L. The patient appears confused and falls asleep during the examination. Which of the following is the most likely diagnosis?

A 32-year-old woman comes to the physician because of a 3-month history of headaches. Two weeks ago, she moved to an area women’s shelter because her husband had been hitting her. She takes no medications. Physical examination shows multiple bruises in different stages of healing over the face, neck, and trunk. There is circumferential bruising on both sides of the neck. Which of the following statements is most appropriate to gather pertinent information from this patient?

A 10-year-old boy with spinal muscular atrophy is brought to the emergency department because of a 2-week history of nausea and vomiting. He also has had decreased appetite during this period. One week prior to his illness, his entire family had similar symptoms which improved after a few days. Physical examination shows decreased skin turgor, dry mucous membranes, and diffuse muscle weakness and atrophy. Laboratory studies show:

Which of the following biochemical cycles in the liver is most likely providing this patient’s brain with an energy source at this time?

A 15-year-old girl is brought to the physician by her mother because of a 1-year history of increasingly severe abdominal pain that occurs periodically, but not regularly. Physical examination shows abdominal tenderness that is most acute around the umbilicus. The physician suspects ectopic endometrial tissue, and an operation is scheduled. During the operation, an 8-cm piece of tissue is removed from the ileal region on the antimesenteric side of the bowel approximately 90 cm proximal to the ileocecal junction. The patient’s pain resolves postoperatively. The ectopic tissue was most likely located in which of the following structures?

In a healthy 25-year-old man with a normal diet and fluid intake, which of the following substances filtered by the kidney has the greatest fraction excreted in urine?

A 2-year-old boy is brought to the physician because of a 6-month history of failure to thrive. Cardiac examination shows a grade 4/6 systolic murmur caused by increased pulmonic flow, followed by a fixed, widely split S2. Echocardiography shows hypertrophy of the right atrium, right ventricle, and pulmonary arteries. This patient most likely has which of the following congenital cardiac anomalies?

A 78-year-old man comes to the physician with his daughter because of low back pain for 3 months. He underwent a bilateral orchiectomy 2 years ago for prostate cancer. He speaks French fluently, but he knows only a few English words. His daughter offers to serve as an interpreter. After thanking the daughter for her offer, it is most appropriate for the physician to state which of the following?

A 45-year-old man comes to the physician because of progressively worsening, constant pain in his left thigh over the past 3 months. He is a long-distance runner. An x-ray of the femur shows thickening of the diaphysis and disruption of the cortex with focal areas of increased calcification. A glistening mass containing several cysts is surgically excised. A photomicrograph of tissue from the mass is shown. Which of the following is the most likely diagnosis?

A 68-year-old man comes to the physician’s office because of the sudden onset of loss of vision in the left eye. There is no pain. He has a 15-year history of well controlled type 2 diabetes mellitus and hypertension. He had a myocardial infarction 2 years ago. Temperature is 36.7°C (98.1 °F), pulse is 72/min, respirations are 16/min, and blood pressure is 200/100 mm Hg. Funduscopic examination shows a pale, white retina with retinal artery narrowing and decreased filling of the retinal veins. Which of the following is the most likely cause of the loss of vision?

A 55-year-old woman comes to the physician because of a 3-year history of weakness, fatigue, decreased appetite, and constipation. She says that she did not receive medical treatment for these symptoms because she did not have health insurance until recently. She was admitted to the hospital 1 year ago because of renal calculi, but she could not afford to pay for follow-up care. X-rays at that time were indicative of osteitis fibrosa cystica. Vital signs are within normal limits. Physical examination shows bony tenderness and muscle strength of 4/5 with decreased tone. Serum studies show an increased calcium concentration and a decreased phosphorus concentration. Which of the following is the most likely diagnosis?

A 58-year-old woman comes to the office for a follow-up examination 2 days after she noticed a lump in her left breast on self-examination. Menopause occurred 6 years ago. Physical examination shows a 1-cm, poorly circumscribed, nodular mass in the left periareolar tissue. Mammography shows an ill-defined density suspicious for malignancy. A needle biopsy specimen is nondiagnostic. Microscopic examination of the excised mass shows a large dilated space lined by glandular epithelium and surrounded by a dense lymphoplasmacytic inflammatory infiltrate; the associated lumen is filled with lipid-laden macrophages and amorphous debris. Which of the following is the most likely diagnosis?

A 76-year-old woman comes to the physician because of a 2-day history of abdominal pain, nausea, and vomiting. Her temperature is 38.9°C (102°F), pulse is 88/min, respirations are 26/min, and blood pressure is 117/79 mm Hg. Abdominal examination shows tenderness of the left lower quadrant. Laboratory studies show a leukocyte count of 22,500/mm3 with 8% bands. ACT scan of the abdomen shows an abscess in the left lower quadrant. The patient undergoes drainage of the abscess and a partial sigmoidectomy for a ruptured diverticulum. Neutrophil activation in this patient is directed, in part, by bacterial lipopolysaccharides. As a result, these lipopolysaccharides most likely bind to which of the following neutrophil receptor types?

A bronchial smooth muscle preparation is maintained in a tissue bath. Bioactive substances can be applied to the smooth muscle, and contraction and relaxation of the smooth muscle can be measured. The results of administration of thromboxane A2 (TXA2) and prostaglandin E2 (PGE2) are shown. Which of the following is the most likely mechanism of the effect of application of the PGE2?

Histone acetyltransferases catalyze the acetylation of lysine residues in the amino-terminal tails of histones. Which of the following is the most likely effect of this covalent modification on chromatin structure?

A 52-year-old man with chronic renal failure receives a cadaveric kidney transplant. Postoperatively, he is given cyclosporine for immunosuppressive therapy. Six weeks after the operation, he develops hypoxemia with an arterial Po2 of 40 mm Hg. A photomicrograph of a transbronchial biopsy specimen is shown. Which of the following pathophysiologic mechanisms best explains this patient’s hypoxemia?

A 3-year-old girl is brought to the physician because of a 2-week history of easy bruisability. She has had frequent, large, foul-smelling stools for 3 months. She was found to have cystic fibrosis at the age of 18 months. She is below the 3rd percentile for height and weight. Physical examination shows several ecchymotic areas over the extremities. This patient most likely has a deficiency of which of the following?

A 65-year-old man who is a veteran comes to the office because of a 3-month history of increasingly severe pain of his low back. He has no history of major medical illness. Physical examination shows point tenderness over the spine at the level of L5. Laboratory studies show:

X-rays of the spine show a compression fracture at L5 and four lytic lesions in the iliac crest. Examination of a bone marrow biopsy specimen confirms the diagnosis and the presence of a chromosomal translocation. Treatment with a chemotherapeutic regimen, including a proteasome inhibitor, is started. Which of the following is most likely to occur in the affected cells as a result of this therapy?

To investigate the association between cellular telephone use and the development of brain tumors, 782 patients with brain tumors are compared with 799 age- and sex-matched patients who do not have a malignancy. The results of the study find that there was no increase in the odds ratio for cellular telephone use in patients who had brain cancer compared with those who did not have cancer. Which of the following best describes this study design?

A 28-year-old white woman comes to the physician for a follow-up examination. Two days ago, she was discharged from the hospital after being treated for a hypertensive crisis with seizure. She has no other history of major medical illness. Current medications include a (3-adrenergic blocker, dihydropyridine calcium channel blocker, and a centrally acting agent stimulating presynaptic a2 receptors. Her temperature is 37°C (98.6°F), pulse is 54/min, and blood pressure is 182/100 mm Hg. Physical examination shows trace ankle edema. Cardiac examination shows an S4 gallop. A soft abdominal bruit is heard left of the umbilicus. Abdominal ultrasonography shows normal-sized kidneys and a normal liver, spleen, pancreas, and gallbladder; a small left ovarian cyst is noted. Laboratory studies show that complete blood count, serum electrolyte concentrations, and renal and liver function test results are within the reference ranges. Which of the following is the most likely cause of this patient’s secondary hypertension?

A 45-year-old man comes to the physician because of a 6-month history of progressive shortness of breath with exertion. He has a history of frequent nosebleeds since adolescence. He is 178 cm (5 ft 10 in) tall and weighs 79 kg (175 lb); BMI is 25 kg/m2. Physical examination shows the findings in the photographs. Inhaled albuterol does not improve his symptoms. Which of the following is the most likely cause of this patient’s condition?

A 63-year-old woman comes to the physician because of a 5-day history of shortness of breath and swollen legs. Her respirations are 25/min and labored, and blood pressure is 130/50 mm Hg. She has a large subclavian arteriovenous fistula caused by a stab wound to the left supraclavicular area 15 years ago. Physical examination shows 2+ edema of the lower extremities. Which of the following findings is most likely in this patient?

A 52-year-old woman with type 2 diabetes mellitus and hypercholesterolemia comes to the physician for a follow-up examination. Since her diagnosis 14 months ago, she has been exercising regularly, eating healthier foods, and taking omega-3 fatty acid supplements. She has had a 9-kg (20-lb) weight loss during this period. She is 168 cm (5 ft 6 in) tall and now weighs 62 kg (137 lb); BMI is 22 kg/m2. Physical examination shows no abnormalities. Serum studies show decreased glucose and cholesterol concentrations since her last visit 2 months ago. She says, “I’m glad everything is fine, but I am worried that I won’t be able to stay on this regimen and that my health will get worse.” Which of the following best describes this patient’s stage of change?

An 81-year-old woman is admitted to the hospital because of a massive pulmonary embolism from a deep venous thrombosis. Her platelet count is 160,000/mm3. Appropriate pharmacotherapy is started. One week later, her platelet count is 55,000/mm3. The thrombocytopenia was most likely caused by a drug with which of the following mechanisms of action?

A 25-year-old man comes to the physician because of sores on his penis for 2 days. Physical examination shows vesicular and ulcerative lesions on the penis. Drug therapy is initiated. One week later, his symptoms resolve. Which of the following enzymes is required to activate the most commonly prescribed drug?

An 18-year-old man is brought to the emergency department 30 minutes after sustaining injuries in a motor vehicle collision. He has abdominal pain. His pulse is 120/min, and blood pressure is 80/60 mm Hg. Physical examination shows multiple contusions over the trunk and abdominal tenderness. A chest x-ray shows multiple rib fractures. A CT scan of the abdomen is shown. Which of the following organs is most likely injured in this patient?

A 52-year-old man who is in the hospital with malignant hypertension, heart failure, and a mild renal impairment is being treated with captopril, nitroprusside, and furosemide. After 48 hours of therapy, his blood pressure is controlled, but he develops metabolic acidosis, lethargy, and difficulty breathing. Which of the following substances most likely caused these adverse effects in this patient?

A 35-year-old woman and her 35-year-old husband come to the physician for genetic counseling after their son is diagnosed with a rare metabolic disease. Assuming Hardy- Weinberg equilibrium, the physician tells the parents about the incidence and carrier frequency of this disorder. Which of the following is most likely to disturb the Hardy- Weinberg equilibrium of this disorder?

A 7-year-old boy is about to undergo an appendectomy. An intravenous catheter needs to be inserted, but the patient is extremely fearful of being stuck with a needle while awake. The most appropriate anesthesia administered by mask to anesthetize this patient quickly would have which of the following characteristics?

Receptors for all of the members of the steroid hormone family share which of the following features?

A 57-year-old woman comes to the physician because of chronic shortness of breath and swollen legs for 2 months. Her respirations are 12/min. Physical examination shows edema of the lower extremities. Auscultation of the chest shows each cardiac systole associated with a thrusting impulse palpable along the left sternal border, most prominently in the third, fourth, and fifth intercostal spaces. Which of the following disorders is the most likely cause of this cardiac finding?

A 34-year-old man has a herniated lumbar intervertebral disc. Laminectomy and removal of the herniated fragment are scheduled. Which of the following labeled sites on the normal lumbar axial CT scan shown is the most appropriate location for surgical entrance to the neural canal?

An 18-month-old boy is brought to the emergency department because of lethargy for 3 hours. He has not eaten well for the past 24 hours. He had cardiorespiratory arrest associated with hypoglycemia after an episode of diarrhea 10 months ago. On arrival, he is unresponsive. His temperature is 37°C (98.6°F), pulse is 140/min, respirations are 25/min, and blood pressure is 100/60 mm Hg. Physical examination shows hepatomegaly. Laboratory studies show:

Following intravenous administration of glucose, he becomes responsive. Medium-chain triglycerides are administered daily for 6 months. At a follow-up examination, his serum glucose concentration is within the reference range. A deficiency of which of the following enzymes is the most likely cause of these findings?

A newborn delivered at 28 weeks’ gestation is in severe respiratory distress. The immature alveoli of this newborn’s lungs have a diminished ability to serve as sites of effective gas exchange. An increase in which of the following parameters best explains this finding?

A 49-year-old man comes to the physician because of a 1-month history of lower abdominal pain and increased urge to defecate. Capability to prevent spontaneous defecation is intact. He sustained nondisplaced multiple fractures of the pelvis 2 months ago in a motor vehicle collision. He initially was on bed rest with pelvic support, progressed to non¬weight-bearing crutch walking, and now is partial weight bearing with crutches. Medications now include naprosyn three times daily. Physical examination shows an anal wink. Neurologic examination shows intact rectal sphincter tone. This patient is able to sustain cortical control over defecation via which of the following nerves?

A 22-year-old woman who has β-thalassemia has a β-hemoglobin allele that carries a single base substitution (A → G) at the splice acceptor site of the second intron. Which of the following events in expression of the β-hemoglobin gene is most likely to be impaired by this mutation?