Histologic Finding in Acute Urticaria Following Penicillin V
The most likely finding on biopsy of these urticarial lesions is dermal edema (Option C).
Understanding the Clinical Presentation
This patient developed acute pruritic urticaria one day after starting penicillin V for confirmed streptococcal pharyngitis. The sudden onset of a pruritic rash following antibiotic initiation strongly suggests an IgE-mediated immediate hypersensitivity reaction 1.
- Urticaria typically manifests as raised, erythematous, pruritic wheals that appear suddenly and can occur within hours to days of drug exposure 1.
- The timing (one day after starting penicillin) is consistent with an immediate-type drug reaction, though delayed urticaria can occur up to several days into therapy 1.
Histopathologic Features of Urticaria
Dermal edema is the hallmark histologic finding in urticaria:
- Urticarial lesions are characterized by dermal edema with separation of collagen bundles due to fluid accumulation in the superficial and mid-dermis 1.
- There is typically a sparse perivascular lymphocytic infiltrate with occasional eosinophils, but the predominant feature is interstitial edema 1.
- The epidermis remains intact without significant changes—no acantholysis, spongiosis, or dyskeratosis 1.
Why Other Options Are Incorrect
- Acantholysis (Option A): Loss of intercellular connections between keratinocytes, seen in pemphigus and other blistering disorders—not characteristic of urticaria 1.
- Acanthosis (Option B): Epidermal thickening seen in chronic conditions like psoriasis or lichen planus—not a feature of acute urticaria 1.
- Dyskeratosis (Option D): Abnormal keratinization seen in conditions like actinic keratosis or squamous cell carcinoma—not present in urticaria 1.
- Epidermal spongiosis (Option E): Intercellular edema within the epidermis characteristic of eczematous dermatitis—urticaria primarily affects the dermis, not the epidermis 1.
Clinical Management Considerations
This patient requires immediate discontinuation of penicillin V and alternative antibiotic therapy:
- For patients with immediate hypersensitivity reactions to penicillin (urticaria, angioedema within hours to days), all β-lactam antibiotics including cephalosporins should be avoided due to up to 10% cross-reactivity risk 1, 2.
- Clindamycin 300 mg orally three times daily for 10 days is the preferred alternative, with only ~1% resistance among Group A Streptococcus in the United States 2, 3.
- Azithromycin 500 mg once daily for 5 days is acceptable but has 5-8% macrolide resistance rates 2, 3.
Symptomatic management of the urticaria:
- Oral antihistamines (H1-blockers such as cetirizine or loratadine) should be initiated for pruritus control 1.
- The urticaria typically resolves within 24-48 hours after discontinuing the offending drug, though recurrent hives may persist for 1-2 days 1.
- Long-acting antihistamines may be continued for several days if needed 1.
Important Pitfall to Avoid
Do not rechallenge with any β-lactam antibiotic in this patient. The development of urticaria within 24 hours of starting penicillin V indicates immediate hypersensitivity, and future exposure to penicillins or cephalosporins carries significant risk of more severe reactions including anaphylaxis 1, 2.