Dermatology MCQ - Inflammatory Dermatoses - Dermatophytide

A 25-year-old man presents with an acute, intensely pruritic, vesicular eruption on his fingers and palms. The primary lesion is groups of deep-seated vesicles. On further examination, you identify a scaly, annular, erythematous plaque with a raised border on his groin, which was previously untreated. A potassium hydroxide (KOH) preparation from the groin lesion confirms a dermatophyte infection. What is the most likely explanation for the hand eruption?

A. Pompholyx
B. Allergic Contact Dermatitis
C. Tinea Manuum
D. Id Reaction (Dermatophytide)
E. Dyshidrotic Eczema

Correct Answer: D. Id Reaction (Dermatophytide)

Explanation:

• The presentation is classic for an Id Reaction (Dermatophytide). This is a generalized acute hypersensitivity eruption that occurs at a site distant from the primary focus of a dermatophyte infection. The key to the diagnosis is identifying the primary infectious source (in this case, tinea cruris in the groin) and recognizing the sterile, reactive nature of the distant eruption (the vesicular hand eruption). The Id reaction is thought to be an immunologic response to fungal antigens.

• Why the other options are incorrect:

  • A. Pompholyx & E. Dyshidrotic Eczema: These are essentially synonymous terms for a common eczematous condition characterized by deep-seated vesicles on the hands and feet. While the morphology is identical, an Id reaction is a specific cause of this pattern. The critical differentiating factor is the presence of an active, often inflammatory, distant fungal infection. Without the tinea infection, this would be classified as pompholyx.

  • B. Allergic Contact Dermatitis: This would present with a distribution that corresponds to contact with an allergen. It would not be explained by the presence of a distant fungal infection.

  • C. Tinea Manuum: This is a direct fungal infection of the hand. A KOH preparation from the vesicles on the hand would be negative in an Id reaction, as the vesicles are sterile and do not contain hyphae. A KOH from the hand would be positive in true tinea manuum.

Key Points for Exams

• Pathophysiology: A type IV delayed hypersensitivity reaction to fungal antigens disseminated from a primary site of infection.

• Common Precipitants: The primary infection is often acute, inflamed, or has been recently traumatized. Tinea pedis (athlete's foot) is the most common trigger, but tinea cruris (jock itch) and tinea corporis (ringworm) can also cause it.

• Clinical Features of the Id Reaction:

  • Morphology: Most commonly a vesicular eruption on the hands (pompholyx-like). Can also be papular, eczematous, or resemble pityriasis rosea.

  • Location: Typically symmetrical on the sides of the fingers, palms, and/or soles.

  • Key Point: The id reaction site is sterile; no fungi are present in these lesions.

• Prognosis: Excellent. The id reaction resolves spontaneously once the primary fungal infection is effectively treated.

• Management:

  1. Treat the Primary Infection: This is the definitive treatment. Use appropriate topical and/or oral antifungal agents (e.g., terbinafine, itraconazole) targeted at the original dermatophyte site.

  2. Symptomatic Relief for the Id Reaction:

     • Topical Corticosteroids: To reduce inflammation and pruritus.

     • Cool Compresses: For acute vesiculation.

     • Oral Antihistamines: For pruritus.

     • In severe, widespread cases, a short course of oral corticosteroids may be considered, but treating the infection remains the priority.