The cartilage surrounding the knee supports considerable pressure because of the presence of chondroitin sulfate, which creates a gel-like medium resilient to shock. Which of the following properties of chondroitin sulfate is responsible for this substance occupying a larger volume in solution than in the dehydrated solid?

A 16-year-old girl is brought to the emergency department 45 minutes after her mother found her in her room with an empty bottle of acetaminophen. She has a history of major depressive disorder. Physical examination shows no abnormalities. Which of the following events would contribute directly to hepatic toxicity in this patient?

A 67-year-old man is brought to the physician by his wife because of a 1-year history of progressive difficulty writing and walking. The patient is stooped and talks slowly. Physical examination shows a bland facial expression. There is a fine resting tremor in both hands, but not in the lower extremities. The tremor is not apparent when the patient is moving. When asked to walk, the patient has some difficulty starting and stopping. There is cogwheel rigidity in all four extremities. Testing of postural reflexes shows moderate retropulsion and mild propulsion. Gross and microscopic appearances of brain tissue from a patient with a similar condition are shown. The midbrain on the left is a normal control. The inclusion indicated by the arrow is predominantly composed of which of the following substances?

A 58-year-old man is brought to the emergency department 2 hours after having a generalized tonic-clonic seizure. He has been treated numerous times in the past for seizures caused by alcohol withdrawal. On arrival, he is awake but confused and tremulous. His pulse is 110/min, and blood pressure is 160/90 mm Hg. Physical examination shows bilateral asterixis. Laboratory studies show:

Which of the following is the most likely cause of this patient’s anemia?

A 13-year-old girl grew 7.5 cm (3 in) over the summer. Which of the following most likely accounts for increased intestinal absorption of calcium during this period?

A 12-year-old boy comes to the physician’s office with dependent and periorbital edema, hypoalbuminemia and proteinuria. A biopsy of the kidney is performed. A photomicrograph of a representative glomerulus is shown (periodic acid-Schiff-stained). Which of the following is the most likely diagnosis?

A 40-year-old man with type 2 diabetes mellitus comes to the physician for a follow-up examination. Treatment with glyburide and metformin has been ineffective in controlling his disease. The decision is made to try pioglitazone. This drug produces a beneficial effect through which of the following mechanisms of action?

A study is done to determine the efficacy of St. John’s wort (Hypericum perforatum) for treatment of endogenous depression. One hundred patients with mild depression are randomly assigned to either a treatment group (900 mg of St. John’s wort daily) or a placebo group. Scores on a Hamilton Depression Rating Scale are recorded for each subject before treatment and 4 weeks after the start of the study. Which of the following best describes this study design?

A 3-month-old boy is brought to the physician because of frequent loose stools during the past month. His maternal cousin has similar symptoms. He is exclusively breast-fed. He is below the 3rd percentile for length and weight. Physical examination shows mild dehydration. Laboratory studies show acidic stools that are positive for reducing substances. Following the oral administration of glucose, there is no increase in his serum glucose concentration. Following the oral administration of fructose, his stools become normal. A deficiency in which of the following in intestinal mucosal cells is most likely in this patient?

A 29-year-old woman comes to the physician because of a 4-month history of headaches that occur every few weeks. They consist of unilateral throbbing pain and are accompanied by nausea and photophobia. She has a history of similar episodes, which started with the onset of menarche, but she had been symptom-free for 7 years until now. She says the headaches seem worse now than ever. Physical examination shows no abnormalities. A diagnosis of migraine is made, and sumatriptan is prescribed. The patient then becomes anxious and tells the physician that she expects further evaluation of her symptoms. She says, “How can you know they are nothing dangerous after only examining me?” She refuses the prescription and begins to cry. After empathizing with the patient, which of the following statements by the physician is most appropriate at this time?

A 15-year-old boy has nasal polyps and develops bronchospasms after taking a drug. The drug is most likely to inhibit which of the following enzymes in the biochemical scheme shown?

The table shows survival information for patients who had an operation for a particular form of cancer:

Based on this information, if a patient survives for 3 years, which of the following best represents the probability that he or she will survive for 4 years?

A 47-year-old man comes to the emergency department because of the sudden onset of severe pain of his lower chest and upper abdomen, and dry heaves. He also has a 4-year history of intermittent heartburn. His pulse is 118/min, and blood pressure is 80/50 mm Hg. Physical examination shows epigastric tenderness. A nasogastric tube cannot be passed into the stomach. The diagnosis of gastric volvulus is made, and appropriate treatment is begun. This patient most likely also has which of the following underlying conditions?

A 62-year-old man with a 4-year history of chronic angina pectoris comes to the emergency department because of severe chest and left shoulder pain. Physical examination shows bradycardia. An ECG shows ST-segment elevation in leads II, III, and aVF. Occlusion of blood flow in which of the following arteries is the most likely cause of the findings in this patient?

A 20-year-old woman comes to the physician because of easy bruising for 2 weeks. She had been in good health except for a viral respiratory illness 1 month ago. Laboratory studies show thrombocytopenia. A bone marrow smear is shown. Which of the following is the most likely cause of her thrombocytopenia?

A 30-year-old woman, gravida 1, para 0, at 32 weeks’ gestation comes to the emergency department because of a 2-day history of fever, nausea, and headache. Physical examination shows a uterus consistent in size with a 30-week gestation. Serologic testing prior to pregnancy showed a positive IgG antibody titer to Toxoplasma gondii. Blood cultures grow a small gram-positive rod, which grows as pinpoint β-hemolytic colonies on sheep blood agar. The organism is catalase positive and exhibits tumbling motility. This infection could have been prevented by avoiding which of the following?

A 12-year-old boy is brought to the office by his mother for an annual well-child examination. He has asthma well controlled with inhaled budesonide. The mother says that the boy recently has voiced concern about his height and is bothered by the fact that he is considerably shorter than the other boys on his basketball team. The patient’s father is 182 cm (6 ft) tall and his mother is 168 cm (5 ft 6 in) tall. The patient is 135 cm (4 ft 5 in; 3rd percentile) tall and weighs 32 kg (71 lb; 10th percentile); BMI is 17 kg/m2 (35th percentile). The patient’s growth has remained steady at the 3rd percentile during the past 8 years. Vital signs are within normal limits. Sexual maturity rating is 1 for pubic hair and genital development. The remainder of the physical examination shows no abnormalities. Which of the following is the most likely diagnosis?

A 14-year-old girl is brought to the physician by her mother because of a 2-month history of hair loss on her head. The mother says that her daughter has been trying to imitate the hairstyles of several pop music stars by using various ties, rubber bands, and curlers, but she is not sure whether any chemical treatments have been used. The mother reports that the patient has been sad since her grandmother died unexpectedly 3 months ago. Vital signs are normal. Physical examination shows decreased hair density over several irregular patches temporally and frontally, but no denuded areas. The underlying skin is normal. The remaining hair shafts in these thinned areas are of varying lengths. There is no frontotemporal recession. A photograph of the affected area is shown. Which of the following is the most likely explanation for this patient’s hair loss?

A 95-year-old man who is a resident of a skilled nursing care facility is brought to the emergency department because of a 1-day history of temperatures to 39.4°C (102.9°F), headache, cough, and muscle aches. Several other residents have similar symptoms. Physical examination shows no other abnormalities. Treatment with a neuraminidase inhibitor is begun. This drug most likely will inhibit which of the following processes, therefore decreasing the duration of this patient’s symptoms?

A 3-month-old boy is brought to the office by his mother because of a 1-week history of a lump on the right side of his groin. The mother says that when the patient cries, the bulge increases in size, and when he is quiet, it disappears. Physical examination shows a soft mass in the right groin region that decreases easily. Which of the following best describes the relationship of the hernia sac?

During a study of antibiotic treatment for pityriasis rosea, two randomized groups of patients with this condition are established (Groups A and B). Group A receives erythromycin and hydroxyzine, and Group B receives only hydroxyzine. The number of days that lesions are present in each group is then determined, and the average of days is calculated for each group. Which of the following statistics is most likely to establish the difference between the averages of these two groups?

A study is conducted to determine the effects of drug X on respiratory function. The respiratory tracing of an experimental animal is shown. Drug X was administered at the arrow. Drug X is most likely which of the following?

A 60-year-old woman is running her first marathon [42 km (26 mi)]. She does not drink enough liquids during the race and becomes dehydrated. Which of the following segments or channels will most likely be activated in this woman’s kidney to help maintain hydration homeostasis?

In a recent study of patients with congestive heart failure, the investigators found that these patients showed a diminished capacity to restore low basal calcium concentration during diastole. Function of which of the following is most likely to be altered in these patients?

A 60-year-old woman comes to the physician because of intermittent joint pain during the past 5 years. The pain is abrupt in onset and involves principally her hands, knees, and toes. She has a history of pyelonephritis; there is no history of trauma. Examination of the left knee shows tenderness, swelling, and warmth with dusky, erythematous overlying skin. A photograph of the left hand is shown, along with a photomicrograph of joint fluid aspirate. Which of the following is most likely in excess concentration in the joint fluid of this patient?

A 28-year-old woman undergoes PPD skin testing to determine previous infection with Mycobacterium tuberculosis. Results show a 25-mm, firm area of induration at 48 hours. Analysis of this patient’s lesion is most likely to show a predominance of which of the following cell types?

A 45-year-old man comes to the physician for a follow-up examination 2 weeks after beginning treatment with hydrochlorothiazide for hypertension. He says that he feels faint, light-headed, and dizzy when he gets out of bed and when he rises from a seated position too quickly. His pulse is 75/min, respirations are 12/min, and blood pressure is 130/85 mm Hg while supine. Physical examination shows no abnormalities. Which of the following sets of changes best characterizes changes in the cardiovascular system as this man goes from the supine to the standing position?

A 6-year-old boy is brought to the physician because of a 2-year history of progressive leg stiffness and difficulty walking. Physical examination shows dystonia of the lower extremities. An MRI of the brain shows no abnormalities. A lumbar puncture is done. Cerebrospinal fluid analysis shows a markedly decreased homovanillic acid concentration. A deficiency of which of the following enzymes is the most likely cause of this patient’s symptoms?

A 27-year-old man comes to the emergency department because of a 3-day history of numbness and a 2-day history of increasing weakness in his arms and legs. He had a “bad cold” 2 weeks ago. He tells the physician that his oral examinations for graduate school are in 1 month and that he broke up with his girlfriend 1 week ago. His temperature is 37.1°C (98.8°F). Neurologic examination shows symmetrical weakness that is greater in his legs than in his arms. Muscle stretch reflexes are absent in all four extremities, and vibration sense and proprioception are diminished in his lower extremities. Which of the following is most likely the greatest contributor to his current symptoms?

A 28-year-old woman, gravida 1, para 1, comes to the physician because of a 1-week history of pain with sexual intercourse. She had a spontaneous vaginal delivery 5 weeks ago and is currently breast-feeding. Pelvic examination shows vaginal dryness and atrophy. The most likely cause is a decreased concentration of which of the following hormones?

A 49-year-old man is brought to the emergency department 30 minutes after fainting in the street. He regained consciousness 1 minute after fainting. He says that he has had watery diarrhea during the past 5 days, which has not improved with fasting. He has not changed his diet or travelled overseas recently. His blood pressure is 90/60 mm Hg. Physical examination shows a flushed face and dehydration. Serum studies show a potassium concentration of 2 mEq/L and glucose concentration of 150 mg/dl_. He is admitted to the hospital, and intravenous fluid replacement is started. Over the next day, he passes a stool with a volume of 3.5 L. Treatment with which of the following hormones is most likely to control this patient’s diarrhea?

Which of the following domains depicted in the schematic diagram of an immunoglobulin molecule shown would be altered by isotype switching?

A 33-year-old woman comes to the physician because of fatigue for 2 months; she feels well otherwise. She has a 10-year history of systemic lupus erythematosus treated with prednisone. Physical examination shows no abnormalities. Laboratory studies show:

Which of the following is the most likely cause of this patient’s anemia?

A 26-year-old man comes to the physician because of palpitations, heat intolerance, and a fine hand tremor for the past month. Examination shows an enlarged thyroid gland. Serum thyroxine (T4) concentration is 28 μg/dL, and serum thyroid-stimulating hormone (TSH) concentration is less than 0.03 μU/mL. The most likely cause of his condition is development of antibodies against which of the following?

A 73-year-old man comes to the physician with his wife because of a 3-year history of daytime sleepiness. His wife says that he snores loudly. He is 183 cm (6 ft) tall and weighs 113 kg (250 lb); BMI is 34 kg/m2. His pulse is 84/min, respirations are 18/min, and blood pressure is 175/105 mm Hg. Physical examination shows congested conjunctivae and centripetal obesity. Serum studies show a glucose concentration of 250 mg/dL, creatinine concentration of 2 mg/dL, and uric acid concentration of 10.1 mg/dL. Polysomnography confirms the diagnosis, and treatment with nasal continuous positive airway pressure at night is started. This therapy will most likely cause a decrease in which of the following?

An overweight 50-year-old man comes to the physician because of a 3-day history of intermittent severe abdominal and interscapular pain associated with nausea and vomiting. He has had an 18-kg (40-lb) weight loss during the past 6 months by caloric restriction and exercise. He has not used appetite suppressants. He is in mild distress. Abdominal examination shows tenderness of the right upper quadrant. Laboratory studies show:

Test of the stool for occult blood is negative. Which of the following is the most likely diagnosis?

A 52-year-old woman with type 2 diabetes mellitus has chronic renal failure. She takes no medications other than glipizide. Creatinine clearance is 20% of normal. Which of the following sets of laboratory findings in serum is most likely in this patient?

A 66-year-old woman comes to the physician because of a 3-week history of difficulty getting up from a seated position. Her temperature is 37°C (98.6°F), pulse is 70/min, and blood pressure is 145/95 mm Hg. Physical examination shows global weakness of the proximal muscles of the extremities. Sensation and reflexes are intact. Her fasting serum glucose concentration is 150 mg/dL. A chest x-ray shows a large mass in the hilum of the left lung. Examination of an endobronchial biopsy specimen shows small cell carcinoma of the lung. Increased secretion of which of the following hormones from the lung carcinoma is the most likely cause of the findings in this patient?

A 72-year-old woman with hypertension comes to the physician for a follow-up examination. She has had no fever, shortness of breath, weight loss, or symptoms of gastroesophageal reflux disease. She was admitted to the hospital because of tuberculosis at the age of 23 years. Current medications include aspirin, atenolol, hydrochlorothiazide, and lisinopril. Her pulse is 68/min, and blood pressure is 140/74 mm Hg. The lungs are clear. Cardiac examination shows a grade 2/6, systolic ejection murmur that is best heard along the left sternal border. A chest x-ray shows normal lung fields and a calcified aortic valve. Which of the following best explains these cardiac findings?

A 50-year-old man with a temperature of 38.9°C (102°F) has produced 120 ml of rust-colored sputum over the past 2 days. Which of the following physical findings most clearly indicates a diagnosis of pneumonia in the right lower lobe?

During an experimental study of oxygen consumption in the kidney, experimental animals are ventilated with 100% nitrogen. Cells from which of the following areas of the kidney are most likely to show the first signs of anoxic injury?

A previously healthy 16-year-old girl comes to the physician because of a progressive rash over her cheeks and the bridge of her nose during the past 5 weeks. She also has a 1-week history of stiffness of her fingers in the mornings that gradually resolves during the day. A photograph of the face is shown. Serum studies show an antinuclear antibody titer of 1:1280 and C3 concentration of 71 mg/dL (ISM 00-200). The pathogenesis of this patient’s cutaneous and musculoskeletal symptoms is most similar to that of which of the following conditions?

A 35-year-old African American man comes to the physician for a routine examination. He recently read a pamphlet at a health fair on the importance of screening for skin cancer. He enjoys sailing and usually goes out on his boat every weekend when the weather is nice. He does not use sunscreen, but he states that he does not “burn.” Physical examination shows no abnormalities. This patient is at increased risk for melanoma at which of the following locations?

A 65-year-old man comes to the emergency department because of a 1 -week history of blood in his sputum. A mass is found on a radiograph. Bronchoscopy is planned. In order to pass the bronchoscope through the oropharynx to the lungs without eliciting a gag reflex, the pharynx is anesthetized. The afferent limb of the reflex is most likely to be blocked by anesthesia to which of the following cranial nerves?

Cholestyramine prevents the reabsorption of bile acids from the lumen of the intestine. This decreases serum cholesterol concentrations by which of the following mechanisms?

A 50-year-old man who has had paraplegia for the past 5 years is admitted to the hospital for treatment of a urinary tract infection. Addressing which of the following factors is most likely to prevent the development of decubitus ulcers in this patient?

A 17-year-old boy is brought to the emergency department 15 minutes after he was found unarousable in the high school’s bathroom. Approximately twenty 5-mg oxycodone tablets were found in his pocket. He has a history of using multiple illicit drugs. Which of the following sets of clinical findings is most likely in this patient?

A 70-year-old woman comes to the physician for a follow-up examination. She had breast cancer 5 years ago and underwent partial mastectomy of the left breast at that time. She has received treatment with tamoxifen since then. Physical examination and mammography show no evidence of recurrence. Which of the following mechanisms most likely explains the beneficial effect of tamoxifen in treating this patient?

Patients with prolonged starvation or untreated type 1 diabetes mellitus overproduce ketone bodies. Which of the following is a common factor that is responsible for ketosis in patients with these two conditions?

A 17-year-old boy has had fever, malaise, and a sore throat for 1 week. Laboratory studies show:

A representative leukocyte from a peripheral blood smear is shown. Which of the following is the most likely diagnosis?