Reviewing culture data on the third day of hospitalization, an internal medicine team eval… — USMLE Step 2 CK

Reviewing culture data on the third day of hospitalization, an internal medicine team evaluates a 71-year-old woman admitted for severe sepsis secondary to pyelonephritis. At presentation, she had a temperature of $39.1^\circ\text{C}$ ( $102.4^\circ\text{F}$ ), a blood pressure of $88/54\text{ mmHg}$ , and severe right flank pain. She was fluid-resuscitated and started on empiric intravenous vancomycin and ceftriaxone.

Her past medical history is notable for type 2 diabetes mellitus, hypertension, and a self-reported penicillin allergy, which she describes as 'severe nausea and diarrhea' occurring after taking an antibiotic for a sinus infection 15 years ago.

Currently, the patient is clinically stable. Her temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $128/78\text{ mmHg}$ , and pulse is $78/\text{min}$ . The right costovertebral angle tenderness has resolved. Her white blood cell count has decreased from $18,500/\text{mm}^3$ to $8,200/\text{mm}^3$ .

Both urine and blood cultures drawn at admission are growing *Enterococcus faecalis*. The isolate is susceptible to ampicillin, vancomycin, linezolid, and daptomycin. A transesophageal echocardiogram demonstrates no evidence of valvular vegetations.

Which of the following is the most appropriate next step in this patient's antimicrobial management?

Discontinue all current antibiotics and initiate intravenous ampicillinAnswer
B
Discontinue ceftriaxone and continue intravenous vancomycin
C
Discontinue all current antibiotics and initiate intravenous piperacillin-tazobactam
D
Discontinue all current antibiotics and initiate intravenous daptomycin
E
Discontinue all current antibiotics and initiate oral linezolid

Answer

Discontinue all current antibiotics and initiate intravenous ampicillin.

The correct approach is to discontinue empiric antibiotics and initiate intravenous ampicillin. This patient has *Enterococcus faecalis* bacteremia originating from pyelonephritis. The isolate is ampicillin-susceptible, making ampicillin the targeted, bactericidal drug of choice. A critical step is evaluating her 'penicillin allergy.' Nausea and diarrhea are common medication side effects, not IgE-mediated hypersensitivity reactions (like anaphylaxis, urticaria, or bronchospasm). Therefore, there is no contraindication to administering beta-lactams. Switching to ampicillin represents optimal antimicrobial stewardship by narrowing the spectrum of coverage, effectively treating the infection while minimizing the risk of developing vancomycin-resistant enterococci (VRE) or *Clostridioides difficile* infection.

Step-by-Step Solution

Analyze the culture and susceptibility results.

The infection is caused by Enterococcus faecalis, which is susceptible to ampicillin, vancomycin, linezolid, and daptomycin.

Identifying the organism and its susceptibilities is required to tailor empiric antimicrobial therapy to targeted therapy.

Evaluate the patient's reported penicillin allergy.

The patient reports 'nausea and diarrhea', which is a gastrointestinal intolerance, not a true IgE-mediated hypersensitivity or severe delayed reaction.

Determining if a true allergy exists dictates whether beta-lactam antibiotics can be safely administered.

Apply antimicrobial stewardship principles to select the most appropriate targeted therapy.

Intravenous ampicillin is selected as the narrowest-spectrum, most effective bactericidal agent.

De-escalating to a narrow-spectrum agent like ampicillin minimizes the risk of resistance (e.g., VRE) and toxicity compared to broad-spectrum alternatives like vancomycin or piperacillin-tazobactam.

Key Concept

Patients with a reported penicillin allergy involving only gastrointestinal intolerance can safely receive beta-lactam antibiotics. For susceptible *Enterococcus faecalis* bacteremia, ampicillin is the narrow-spectrum, bactericidal drug of choice.

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