A 16-year-old girl presents to the clinic with concerns about her toenails.

In a healthy adolescent training for a triathlon, the most likely cause of subacute toenail changes is repetitive microtrauma from running—often due to shoes with inadequate toe box space or downhill running—leading to nail plate injury (e.g., subungual hemorrhage/onycholysis and dystrophic discoloration). MCCQE objectives emphasize choosing the most probable diagnosis based on context and initiating the least invasive, most appropriate first-line management. The key intervention is correcting the mechanical cause: ensuring adequate toe space, proper shoe fit, and appropriate nail trimming, which typically prevents progression and allows gradual regrowth over months.

Topical antifungal creams are unlikely to help if the primary issue is trauma and, even in onychomycosis, topical creams generally do not penetrate the nail plate effectively. Hygiene advice and avoiding pools/hot tubs may reduce superficial fungal exposure but does not address the principal driver in this scenario. Biopsy is unnecessary for typical trauma-related nail changes in a low-risk patient without concerning features (rapid growth, pigment extending onto adjacent skin, or mass).