Penicillins and beta-lactamase inhibitors — MCQs

A pharmaceutical company has modified one of its existing antibiotics to have an improved toxicity profile. The new antibiotic blocks protein synthesis by first entering the cell and then binding to active ribosomes. The antibiotic mimics the structure of aminoacyl-tRNA. The drug is covalently bonded to the existing growing peptide chain via peptidyl transferase, thereby impairing the rest of protein synthesis and leading to early polypeptide truncation. Where is the most likely site that this process occurs?

An 18-year old college freshman presents to his university clinic because he has not been feeling well for the past two weeks. He has had a persistent headache, occasional cough, and chills without rigors. The patient’s vital signs are normal and physical exam is unremarkable. His radiograph shows patchy interstitial lung infiltrates and he is diagnosed with atypical pneumonia. The patient is prescribed azithromycin and takes his medication as instructed. Despite adherence to his drug regimen, he returns to the clinic one week later because his symptoms have not improved. The organism responsible for this infection is likely resistant to azithromycin through which mechanism?

A 64-year-old female with type 2 diabetes mellitus comes to the physician because of a 1-week history of painful red swelling on her left thigh. Examination shows a 3- x 4-cm, tender, fluctuant mass. Incision and drainage of the abscess are performed. Culture of the abscess fluid grows gram-positive, coagulase-positive cocci that are resistant to oxacillin. Which of the following best describes the mechanism of resistance of the causal organism to oxacillin?

A 21-year-old woman comes to the physician because of a 4-day history of abdominal cramps and bloody diarrhea 5 times per day. Her symptoms began after she ate an egg sandwich from a restaurant. Her vital signs are within normal limits. Physical examination shows diffuse abdominal tenderness. Stool culture shows gram-negative rods that produce hydrogen sulfide and do not ferment lactose. Which of the following effects is most likely to occur if she receives antibiotic therapy?

A 51-year-old man is admitted to the hospital because of a 2-day history of fever, nausea, and abdominal pain. His temperature is 39.4°C (102.9°F) and pulse is 106/min. Physical examination shows tenderness in the right upper quadrant. Blood cultures grow nonhemolytic, gram-positive cocci that grow in hypertonic saline. Antibiotic sensitivity testing of the isolated organism shows that gentamicin has a minimum inhibitory concentration (MIC) of 16 μg/mL. The addition of ampicillin, which has an MIC of 2 μg/mL alone, decreases the MIC of gentamicin to 0.85 μg/mL. The decrease in the MIC of gentamicin with the addition of ampicillin is most likely due to which of the following mechanisms?

An investigator is studying a strain of bacteria that retains a blue color after crystal violet dye and acetone are applied. The bacteria are inoculated in a petri dish containing hypotonic saline. After the addition of an antibiotic, the bacteria swell and rupture. This antibiotic most likely belongs to which of the following classes?

A 14-year-old boy is brought to the emergency department by his parents for joint pain following the acute onset of a diffuse, pruritic rash for the past 24 hours. A week ago, he was diagnosed with pharyngitis after returning home from summer camp and is currently taking antibiotics. There is no family history of serious illness. His temperature is 38.5°C (101.3°F), pulse is 90/min, and blood pressure is 110/70 mm Hg. Physical examination shows periorbital edema, generalized lymphadenopathy, and well-circumscribed, erythematous, confluent skin lesions of variable sizes up to several centimeters in width over his entire body. There is pain on passive movement of wrists and ankle joints bilaterally. Urine dipstick shows 1+ proteinuria. There is no hematuria. Which of the following is the most appropriate next step in management?

A 26-year-old woman presents to her primary care physician because she has been experiencing occasional fevers and chills for the last 3 weeks. She says that the fevers have been accompanied by abdominal pain and increased vaginal discharge. On presentation her temperature is 101.0°F (38.3°C), blood pressure is 113/75 mmHg, pulse is 105/min, and respirations are 12/min. On physical exam she is found to have tenderness over the lower abdominal quadrants, and speculum exam shows uterine inflammation as well as a retained intrauterine device. The most likely cause of this patient's symptoms should be treated with an antibiotic with which mechanism of action?

A hospitalized 45-year-old man has had mild flank pain since awakening 3 hours ago. He also reports a new generalized rash. Two weeks ago, he was diagnosed with pulmonary tuberculosis. Current medications include isoniazid, pyrazinamide, rifampin, ethambutol, and pyridoxine. His temperature is 38.3°C (100.9°F), pulse is 74/min, and blood pressure is 128/72 mm Hg. Examination of the skin shows diffuse erythema with confluent papules. There is no costovertebral angle tenderness. Laboratory studies show: Leukocyte count 9,800/mm3 Segmented neutrophils 59% Bands 3% Eosinophils 4% Lymphocytes 29% Monocytes 5% Serum Urea nitrogen 25 mg/dL Creatinine 1.9 mg/dL Urine WBC 8–10/hpf Eosinophils numerous RBC 5–6/hpf RBC casts negative WBC casts numerous In addition to intravenous fluid resuscitation, which of the following is the most appropriate next step in management?

An 8-year-old boy is brought to the emergency department by his parents because of sudden onset of abdominal pain beginning an hour ago. The parents report that their son has also had an episode of dark urine earlier that morning. Three days ago, he was diagnosed with a urinary tract infection and was treated with trimethoprim-sulfamethoxazole. He emigrated from Liberia to the US with his family 3 years ago. There is no personal history of serious illness. His immunizations are up-to-date. Vital signs are within normal limits. Examination shows diffuse abdominal tenderness and scleral icterus. The spleen is palpated 1–2 cm below the left costal margin. Laboratory studies show: Hemoglobin 10 g/dL Mean corpuscular volume 90 μm3 Reticulocyte count 3% Serum Bilirubin Total 3 mg/dL Direct 0.5 mg/dL Haptoglobin 20 mg/dL (N=41–165 mg/dL) Lactate dehydrogenase 160 U/L Urine Blood 3+ Protein 1+ RBC 2–3/hpf WBC 2–3/hpf Which of the following is the most likely underlying cause of this patient's symptoms?