A 42-year-old woman with a history of penicillin anaphylaxis presents with a methicillin-resistant Staphylococcus aureus (MRSA) skin abscess requiring systemic antibiotic therapy. Which is the drug of choice?

Explanation

## Drug of Choice for MRSA Infection (Beta-Lactam Allergy) **Key Point:** Vancomycin is the first-line agent for MRSA infections in patients with documented beta-lactam allergy. It is a glycopeptide antibiotic with excellent activity against all staphylococcal strains, including MRSA. **High-Yield:** Standard vancomycin dosing for skin/soft tissue MRSA infection: - **IV vancomycin: 15–20 mg/kg Q8–12H** (target trough 15–20 μg/mL) - Oral vancomycin is NOT absorbed systemically and is reserved for Clostridioides difficile colitis only - For uncomplicated skin infections, oral alternatives include linezolid or doxycycline if susceptible **Clinical Pearl:** In this case, because the patient has **penicillin anaphylaxis**, cephalosporins (including cefazolin) are contraindicated due to ~1–3% cross-reactivity risk with penicillin. Vancomycin avoids this risk entirely and is the safest choice. **Mnemonic:** **MRSA = Vancomycin (or Linezolid/Doxycycline for oral)** - **M**ethicillin-**R**esistant → needs non-beta-lactam - **S**taphylococcus → glycopeptide or oxazolidinone - **A**ureus → vancomycin is gold standard ## Treatment Algorithm for MRSA Infection ```mermaid flowchart TD A[MRSA Infection Confirmed]:::outcome --> B{Beta-lactam Allergy?}:::decision B -->|No allergy| C[Nafcillin/Oxacillin if MSSA]:::action B -->|MSSA| D[Antistaphylococcal Penicillin]:::action B -->|MRSA + No allergy| E[Vancomycin or Linezolid]:::action B -->|Anaphylaxis risk| F[Vancomycin IV]:::action F --> G[Monitor trough 15-20 mcg/mL]:::action E --> H[Assess severity]:::decision H -->|Severe/Systemic| I[IV Vancomycin]:::action H -->|Mild/Localized| J[Oral Linezolid or Doxycycline]:::action ``` ## Comparison: MRSA Treatment Options | Agent | Route | Indication | Advantage | Disadvantage | | --- | --- | --- | --- | --- | | Vancomycin | IV | MRSA (all sites) | Excellent tissue penetration; no cross-reactivity | Requires TDM; nephrotoxicity risk | | Linezolid | IV/PO | MRSA skin/soft tissue | Good oral bioavailability; no TDM needed | Expensive; thrombocytopenia with prolonged use | | Doxycycline | PO | MRSA skin/soft tissue (susceptible) | Cheap; good oral absorption | Photosensitivity; contraindicated in pregnancy | | Cefazolin | IV | MSSA only | Beta-lactam advantage | **CONTRAINDICATED in penicillin anaphylaxis** | **Warning:** Cephalosporins carry a 1–3% cross-reactivity risk with penicillin anaphylaxis and should be avoided in this patient. Amoxicillin-clavulanate is a penicillin derivative and is absolutely contraindicated. [cite:KD Tripathi 8e Ch 54]