A 22-year-old woman from Mumbai presents with a 5-day history of sore throat, fever (38.8°C), and progressive difficulty swallowing. She has bilateral tonsillar enlargement with yellowish exudate, cervical lymphadenopathy, and mild hepatosplenomegaly. Monospot test is positive. She is given amoxicillin 500 mg TDS. Within 24 hours, she develops a diffuse maculopapular rash over the trunk and extremities. What is the most likely explanation for the rash?

Explanation

## Ampicillin/Amoxicillin Rash in Infectious Mononucleosis ### Clinical Context **Key Point:** The ampicillin rash (or amoxicillin rash) in infectious mononucleosis (IM) is a well-recognized, non-allergic drug reaction occurring in 80–90% of IM patients who receive penicillins or cephalosporins. ### Diagnostic Clues in This Case | Finding | Significance | |---------|---------------| | **Positive Monospot** | Confirms infectious mononucleosis (EBV) | | **Bilateral tonsillar exudate** | Classic for IM (not typical of streptococcal pharyngitis alone) | | **Hepatosplenomegaly** | Systemic EBV infection | | **Rash timing: 24 hrs post-amoxicillin** | Temporal relationship with penicillin administration | | **Rash morphology: maculopapular** | Consistent with ampicillin rash (not urticarial) | ### Pathophysiology of Ampicillin Rash in IM ```mermaid flowchart TD A[EBV Infection]:::outcome --> B[Polyclonal B-cell Activation]:::action B --> C[Increased Circulating Immune Complexes]:::action C --> D[Altered Drug Metabolism/Immune Tolerance]:::action D --> E[Amoxicillin/Ampicillin Administration]:::action E --> F[Immune Complex Deposition in Skin]:::action F --> G[Maculopapular Rash]:::outcome H[NOT IgE-mediated Anaphylaxis]:::urgent -.->|Key distinction| G ``` **High-Yield:** The rash is thought to result from: 1. Polyclonal B-cell activation by EBV → increased circulating immune complexes 2. Altered drug metabolism or reduced immune tolerance in IM 3. Immune complex deposition in skin vasculature → Type III hypersensitivity ### Why It's NOT True Penicillin Allergy **Clinical Pearl:** This rash is **NOT** a true IgE-mediated allergy. Patients with ampicillin rash in IM can usually tolerate penicillins in the future without reaction. The rash is benign, self-limited, and does not require drug discontinuation in most cases. | Feature | Ampicillin Rash in IM | True Penicillin Allergy | |---------|----------------------|------------------------| | **Onset** | 24–96 hrs (delayed) | Minutes to hours | | **Morphology** | Maculopapular | Urticarial, angioedema | | **Mechanism** | Immune complex (Type III) | IgE-mediated (Type I) | | **Prognosis** | Benign, resolves in days | Risk of anaphylaxis | | **Future tolerance** | Usually tolerate penicillins | Avoid all penicillins | ### Management 1. **Continue amoxicillin** (unless severe systemic symptoms develop) 2. Symptomatic care: antihistamines, topical corticosteroids if pruritic 3. Rash typically resolves within 5–7 days despite continuing the drug 4. **Do NOT label as penicillin-allergic** in medical record **Warning:** Avoid mislabeling this patient as penicillin-allergic, as it limits future antibiotic options and increases unnecessary use of broad-spectrum agents. [cite:Harrison 21e Ch 187 & 303; Robbins & Cotran 10e Ch 6]