onychomycosis

Onychomycosis, also called tinea unguium, is a chronic fungal infection of the nail plate and bed. Dermatophytes (Trichophyton rubrum most commonly, also T. mentagrophytes) cause the majority of cases; Candida species and non-dermatophyte molds (e.g., Scopulariopsis) are less common causes, with Candida more often implicated in fingernail infections, especially in immunocompromised patients or those with chronic paronychia. Risk factors include advanced age, diabetes mellitus, peripheral vascular disease, immunosuppression, tinea pedis, nail trauma, and occlusive footwear. Clinically, it presents as distal lateral subungual onychomycosis (most common pattern)—nail thickening (hyperkeratosis), yellow-white-brown discoloration, onycholysis (separation of nail from nail bed), and brittleness. Other patterns include white superficial onychomycosis and proximal subungual onychomycosis (the latter is a classic sign of HIV/immunosuppression). Diagnosis requires confirmation before treatment given the length and cost of therapy: KOH preparation showing hyphae, fungal culture, PAS staining of nail clippings (most sensitive), or PCR. Differential diagnosis includes psoriasis, lichen planus, and traumatic nail dystrophy. Treatment options include oral antifungals (terbinafine is first-line; itraconazole is an alternative), which require monitoring of liver function due to hepatotoxicity risk, or topical antifungals (efinaconazole, ciclopirox) for milder cases or when systemic therapy is contraindicated. Cure rates are modest and recurrence is common. This is a testable topic connecting dermatology, microbiology (dermatophyte biology), and pharmacology (antifungal mechanisms and monitoring).