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?

Question

Accepted Answer

B. Vancomycin. ## 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]

Answer

A. Erythromycin

Answer

C. Amoxicillin-clavulanate

Answer

D. Cefazolin

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)

Treatment Algorithm for MRSA Infection

Comparison: MRSA Treatment Options

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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