An 82-year-old man is evaluated in the intensive care unit for worsening oxygenation and p… — USMLE Step 3

An $82$ -year-old man is evaluated in the intensive care unit for worsening oxygenation and persistent fever. He was admitted $4$ days ago for severe community-acquired pneumonia and required endotracheal intubation on hospital day $2$ due to acute respiratory failure. His empiric antimicrobial regimen includes intravenous vancomycin and cefepime. Today, his temperature is $38.9^\circ\text{C}$ ( $102.0^\circ\text{F}$ ), blood pressure is $105/65\text{ mmHg}$ , and heart rate is $112\text{/min}$ . Endotracheal aspirate cultures drawn on admission have grown *Staphylococcus aureus*. Susceptibility testing reveals resistance to oxacillin and a vancomycin minimum inhibitory concentration (MIC) of $4.0\text{ }\mu\text{g/mL}$ .

Pertinent laboratory results:
- Leukocyte count: $18,500\text{/mm}^3$
- Serum creatinine: $2.3\text{ mg/dL}$ (baseline $1.1\text{ mg/dL}$ )

His medical history is significant for major depressive disorder and atrial fibrillation, for which he takes home medications including escitalopram, metoprolol, and apixaban via an orogastric tube. A decision is made to discontinue vancomycin and initiate an alternative agent for the methicillin-resistant *Staphylococcus aureus* (MRSA) infection. Which of the following is the most appropriate targeted pharmacologic intervention?

Intravenous ceftarolineAnswer
B
Intravenous linezolid
C
Intravenous daptomycin
D
Increased dose of intravenous vancomycin
E
Intravenous piperacillin-tazobactam

Answer

Intravenous ceftaroline

The correct intervention is intravenous ceftaroline. Ceftaroline is an advanced-generation cephalosporin with unique affinity for penicillin-binding protein 2a (PBP2a), granting it robust bactericidal activity against methicillin-resistant Staphylococcus aureus (MRSA). The patient has a severe MRSA pneumonia caused by a vancomycin-intermediate strain (VISA, defined as a vancomycin MIC of 4-8 μg/mL). Therefore, vancomycin must be discontinued. Ceftaroline has excellent pulmonary penetration and is a guideline-concordant alternative for MRSA pneumonia. While it requires dose adjustment for his acute kidney injury, it does not carry the absolute contraindications present with the other options.

Step-by-Step Solution

Interpret the microbiological data.

The patient has a MRSA pneumonia with a vancomycin MIC of 4.0 μg/mL.

An MIC ≥ 4.0 μg/mL defines Vancomycin-Intermediate Staphylococcus aureus (VISA). Vancomycin therapy will fail and must be discontinued, prompting the selection of an alternative anti-MRSA agent.

Evaluate daptomycin as an alternative.

Daptomycin is excluded from consideration.

Daptomycin is irreversibly bound and inactivated by pulmonary surfactant, rendering it useless for pulmonary infections.

Evaluate linezolid against the patient's medication list.

Linezolid is excluded due to a critical drug-drug interaction.

Linezolid is a weak, reversible monoamine oxidase inhibitor (MAOI). The patient is concurrently receiving escitalopram, a selective serotonin reuptake inhibitor (SSRI). Combining these medications creates a high risk for life-threatening serotonin syndrome.

Select the safest, most effective remaining agent.

Ceftaroline is selected.

Ceftaroline is a 5th-generation cephalosporin that uniquely binds PBP2a, conferring bactericidal activity against MRSA (including VISA). It penetrates lung tissue well and lacks the critical interactions or contraindications present in the other options.

Key Concept

Selection of pharmacologic therapy for MRSA pneumonia requires synthesizing resistance data (VISA definition), site-specific pharmacokinetics (daptomycin inactivation by surfactant), and severe drug-drug interactions (linezolid with SSRIs).

Estimated Time:2m 0s

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