A 24-year-old man comes to the physician with painful blisters on the shaft of his penis.  The lesions erupted 2 days ago.  The patient has had 5 lifetime sexual partners and is currently sexually active with one female partner; he uses condoms inconsistently.  Examination shows multiple vesicular lesions on the penis, and Tzanck smear is positive for multinucleated giant cells.  HIV testing is negative.  The patient has had several similar episodes every year for the past 2 years but had been too embarrassed to seek treatment until now.  Which of the following would most likely have prevented recurrence of this patient’s condition?

A 52-year-old man is evaluated for low-grade fever, malaise, abdominal pain, and diarrhea.  The patient underwent a cadaveric renal transplant 6 months ago to treat renal failure from underlying polycystic kidney disease.  He takes maintenance immunosuppressive therapy.  Physical examination shows mild tenderness in the lower abdominal quadrants with no rebound tenderness.  Serum immunosuppressant levels are within therapeutic range, and renal function studies are normal.  Colonoscopy reveals mucosal erythema and ulcerations.  Biopsy shows large cells with inclusion bodies, and a polymerase chain reaction-based test detects viral DNA.  First-line therapy for the infection is initiated.  This patient is at greatest risk of which of the following from the treatment?

An epidemiologic study is conducted after a sudden increase in the rate of severe postoperative infections in a rural hospital.  It is determined that all the patients who developed the infection were operated on with surgical instruments sterilized by a specific autoclave machine.  The machine was found to be inadequately sterilizing the equipment due to a malfunction limiting the temperature to 100 C (212 F) during the 15 minute cycle.  These patients most likely developed infection from which of the following bacteria?

A 66-year-old man comes to the clinic due to numbness and tingling in his hands and feet that developed over the last few weeks.  The patient was healthy until he was diagnosed with pulmonary tuberculosis 3 months ago.  He takes only isoniazid and rifampin.  Temperature is 37.1 C (98.8 F), blood pressure is 126/80 mm Hg, pulse is 90/min, and respirations are 16/min.  Cardiopulmonary examination is unremarkable.  Neurologic examination shows ataxia and decreased pain sensation in the distal extremities.  This patient’s condition is most likely due to which of the following?

A 31-year-old male farm worker comes to the physician complaining of an itchy rash on his chest.  Physical examination reveals an annular and scaling plaque five centimeters in diameter with central clearing on the chest.  KOH preparation of skin scrapings shows branching septate hyphae, and topical application of terbinafine was prescribed.  Which of the following mechanisms of action explains the antifungal activity of this drug?

A boy is delivered via spontaneous vaginal delivery at 39 weeks gestation to a woman, gravida 1 para 0.  The boy is vigorous with a strong cry, requiring no interventions after delivery except for warming and drying.  APGAR scores are 8 and 9 at 1 and 5 minutes, respectively.  Head circumference measures <10th percentile, and the placenta has numerous calcifications.  Serology is consistent with congenital toxoplasmosis.  As part of treatment, he is prescribed a combination of pyrimethamine and sulfadiazine.  Which of the following best describes the reason for using multiple drugs during this patient’s treatment?

A 45-year-old man comes to the emergency department due to 2 days of left knee swelling and pain.  The patient has no significant past medical history apart from an episode of facial palsy 3 months ago.  Six months ago, he went on a hiking trip to New Hampshire but has no other travel history.  The patient is in a long-standing monogamous relationship with his wife and has had no other sexual partners.  Temperature is 37.1 C (98.8 F).  Examination shows left knee joint swelling with no surrounding erythema.  The remainder of the musculoskeletal examination is unremarkable.  There is no heart murmur.  Which of the following might have prevented this patient’s knee condition?

A 13-year-old boy is brought to the clinic by his parents due to a change in behavior.  His special education teacher reported that for the last several weeks he has frequently taken off his shoes to scratch his feet during class.  The patient has a history of learning disability but is otherwise healthy.  Examination shows erythema and excoriations between the toes of both feet.  Microscopy of skin scrapings from the rash is shown in the exhibit.  Which of the following antimicrobial drugs is likely to be the most effective against the pathogen causing this patient’s condition?

A 28-year-old woman comes to the office due to several months of progressive abdominal pain and intermittent diarrhea.  She denies fever or chills but has lost weight due to poor appetite.  The patient works for an organization that installs wood pellet cookstoves in rural villages in Central Africa and has spent much time in the region over the last 5 years.  Temperature is 37.8 C (100 F), blood pressure is 126/72, and pulse is 86.  BMI is 19 kg/m².  Physical examination is normal.  Laboratory results are as follows:

An intestinal biopsy sample is shown in the exhibit.  Which of the following is the most appropriate treatment for this patient?

A 6-month-old boy is evaluated due to cough for 2 weeks.  The patient had a mild runny nose without fever prior to the onset of cough.  His 9-year-old brother had a prolonged cough recently.  The patient has not received vaccinations.  Temperature is 37.1 C (98.8 F), blood pressure is 98/66 mm Hg, and respirations are 30/min.  Oxygen saturation is 98%.  On physical examination, when the patient coughs intermittently, his face turns red.  The lungs are clear to auscultation.  Leukocyte count is 22,000/mm³ (75% lymphocytes).  The most effective pharmacotherapy for this patient’s condition will directly interfere with which of the following cellular processes?

A 62-year-old woman is being evaluated for worsening anemia.  The patient was admitted to the hospital 2 days ago due to acute pyelonephritis.  She has been treated with intravenous ceftriaxone, and her fever and urinary symptoms have gradually improved.  The patient has no history of anemia and has had no symptoms of urinary or gastrointestinal bleeding.  Laboratory results are as follows:

Her antibiotics are changed to a different medication class, and the patient is discharged a few days later.  At a follow-up office visit 2 weeks later, her laboratory abnormalities have resolved.  Which of the following is the most likely cause of this patient’s anemia?

An 8-year-old boy is brought to the office due to worsening thrush and angular cheilitis, which causes difficulty eating.  The patient has a history of chronic mucocutaneous candidiasis and has had recurrent Candida infections of the skin, oral mucosa, and nails since infancy.  He is currently on suppressive therapy with fluconazole, which has been successful in preventing infections until this most recent episode.  Physical examination shows extensive oral mucosal candidiasis.  Scrapings from a lesion are obtained for culture, and intravenous antifungal therapy is initiated.  Sensitivity testing of the colonies that grow in culture show Candida that is resistant to fluconazole.  Which of the following changes in the pathogen best explains this patient’s breakthrough infection?

A 12-year-old boy is brought to the emergency department due to a skin infection.  Temperature is 38.4 C (101.1 F).  Physical examination shows an area of erythema, warmth, and tenderness on his right distal leg.  Laboratory results are notable for leukocytosis.  The patient is started on intravenous nafcillin.  Two days later, he shows limited response to antibiotic therapy.  Light microscopy of pus obtained from the site shows gram-positive cocci in clusters, and sensitivity testing demonstrates little response to nafcillin but good response to vancomycin.  Which of the following is the most likely explanation for nafcillin treatment failure in this patient?

A novel aminoglycoside antibiotic is developed that is intravenously administered and excreted unchanged in urine with an elimination half-life of 4 hours.  Two different dosing regimens are tested in a clinical trial: smaller doses given 3 times a day and a higher dose given once a day.  The total administered drug amount per kilogram of body weight per day (mg/kg/day) is the same in both regimens.  Serum drug concentration is monitored in both groups and is shown in the graph below:

[Blue: once-daily (extended interval) dosing, Green: multiple-daily dosing, MIC: minimum inhibitory concentration]

It is found that both regimens are effective against gram-negative pathogens, with clinical improvement occurring slightly earlier on average with once-daily dosing.  Which of the following best explains the efficacy of once-daily dosing despite the short half-life of this antibiotic?

A 64-year-old man comes to the office for follow-up after a recent hospitalization.  Ten days ago, he was admitted due to fever, dysuria, urinary urgency, and perineal pain.  Evaluation revealed acute prostatitis, and urine culture was positive for Escherichia coli that was sensitive to all tested antibiotics.  He received intravenous ciprofloxacin with improvement in his symptoms and was subsequently discharged on oral ciprofloxacin.  The patient has been taking the antibiotic as prescribed, but over the past 2 days he has had recurrent fever and dysuria.  He takes diltiazem for palpitations, atorvastatin for hyperlipidemia, and an over-the-counter antacid for frequent heartburn.  Temperature is 37.3 C (99.1 F), blood pressure is 118/68 mm Hg, and pulse is 90/min.  Physical examination shows a mildly enlarged and tender prostate.  Repeat urine culture grows E coli.  Which of the following is the most likely cause of this patient’s recurrent symptoms?

A 17-year-old girl comes to the office due to worsening acne.  The patient’s acne had been well controlled with oral doxycycline therapy.  However, over the past month, the acne has worsened on her face and back.  The patient was recently diagnosed with iron deficiency anemia due to abnormal uterine bleeding and began taking a combined oral contraceptive and an iron supplement.  Physical examination shows comedones and pustules on her face and upper back.  The remainder of the examination shows no abnormalities.  Which of the following is the most likely cause of this patient’s worsening symptoms?

A 42-year-old woman is hospitalized due to fever and chills after a hemodialysis session.  The patient has a history of end-stage kidney disease due to IgA nephropathy and recently began intermittent dialysis through a tunneled catheter.  Medical history includes depression, for which she takes citalopram.  Temperature is 38.4 C (101.1 F), blood pressure is 130/80 mm Hg, and pulse is 94/min.  There is no erythema or tenderness at the catheter site, and the remainder of the physical examination shows no abnormalities.  Blood cultures are obtained, and empiric vancomycin and ceftazidime are initiated.  While receiving the intravenous vancomycin infusion, the patient reports a burning, itching sensation.  Vital signs are unchanged, but repeat examination shows an erythematous rash involving the face and neck.  She reports no history of drug allergy but has never received these antibiotics.  Which of the following is the most likely underlying cause of this patient’s current condition?

A 26-year-old man is being evaluated for recurrent boils and skin abscesses.  Anterior nares swab culture is performed to determine colonization by the culprit bacteria, which yields gram-positive cocci in clusters.  The bacteria are able to grow in a media containing oxacillin.  PCR testing reveals the pathogen has acquired the mecA gene.  These bacteria are most likely to be resistant to which of the following antibiotics?

A 70-year-old man is diagnosed with a pulmonary nocardia infection.  Medical conditions include hypertension and diet-controlled type 2 diabetes mellitus.  Therapy is started with trimethoprim-sulfamethoxazole for a planned duration of 3-6 months.  This patient is at greatest risk for which of the following drug-induced adverse effects?

A 19-year-old woman comes to the office due to 2 days of dysuria, urinary frequency, and urgency.  The symptoms began a day after having sexual intercourse with her boyfriend.  The patient has no fever, flank pain, nausea, or vomiting.  She has no chronic medical conditions and no known drug allergies.  Vital signs are within normal limits.  Examination shows mild suprapubic tenderness.  Urinalysis reveals many bacteria and white blood cells/hpf.  Which of the following is the most appropriate pharmacotherapy for this patient?