A 61-year-old man presents with thickened toenails. Keratin and debris have accumulated distally and underneath, and the nail has separated from the nail bed. Which of the following is most consistent with this clinical scenario?

Choice A reason: Pseudomonas infections can cause green discoloration of nails, but do not typically cause thickening with distal subungual keratin accumulation as described. Choice B reason: Onychomycosis is a fungal nail infection characterized by thickening, debris accumulation under the nail, and onycholysis (nail separation from the bed), matching the patient’s clinical presentation. Choice C reason: Herpetic whitlow is a viral infection of the finger or toe causing painful vesicles, not thickened nails with subungual debris. Choice D reason: Parasitic infections rarely affect nails in this manner and do not produce the chronic thickening and keratin buildup observed in onychomycosis.