What causes whitening at the base of a toenail or fingernail affected by onychomycosis?
White discoloration at the base (proximal nail fold) of a toenail or fingernail—often referred to colloquially as “white spots near the cuticle”—can have several underlying causes. While onychomycosis (fungal nail infection) is commonly suspected, especially in cases of chronic or progressive nail changes, proximal white discoloration is not typical of classic dermatophyte infections, which more often begin at the distal or lateral nail edges. Instead, a white band or patch extending from the proximal nail fold may suggest proximal subungual onychomycosis (PSO), a less common but clinically significant variant caused most frequently by *Trichophyton rubrum* or, in immunocompromised individuals, by non-dermatophyte molds or yeasts such as *Candida albicans*. PSO warrants prompt evaluation, particularly in patients with HIV, diabetes, or other conditions affecting immune function.
Other important differential diagnoses include psoriatic nail disease, which can present with leukonychia (white nail discoloration), pitting, onycholysis, or oil-drop lesions; lichen planus, which may cause longitudinal ridging and proximal nail matrix involvement; and trauma-induced leukonychia, where minor injury to the nail matrix results in transient white opacities. Systemic conditions—including hypoalbuminemia, renal failure, or zinc deficiency—can also manifest as diffuse or punctate leukonychia, though these typically affect multiple nails symmetrically rather than isolated proximal whitening.
Accurate diagnosis requires clinical assessment combined with laboratory testing: potassium hydroxide (KOH) preparation, fungal culture, and/or nail plate biopsy with periodic acid–Schiff (PAS) staining. Empiric antifungal therapy without confirmation risks unnecessary treatment, drug toxicity, and delayed diagnosis of mimicking conditions. Patients should be advised to avoid nail trauma, maintain foot hygiene, and seek evaluation if changes progress, involve multiple nails, or are accompanied by pain, swelling, or systemic symptoms.