A 75-year-old man sought care from his primary care physician because his “fingernails and toenails [were] all falling off.” He did not feel ill and had no other complaints. His vital signs were unremarkable. He had no history of malignancies, chronic skin conditions, or systemic diseases. His fingernails and toenails were discolored and lifting from the proximal end of his nail beds (FIGURE). One of his great toenails had already fallen off, 1 thumb nail was minimally attached with the cuticle, and the rest of his nails were loose and in the process of separating from their nail beds. There was no nail pitting, rash, or joint swelling and tenderness. The patient reported that while on vacation in Hawaii 3 weeks earlier, he had sought care at an urgent care clinic for a painless rash on his hands and the soles of his feet. At that time, he did not feel ill or have mouth ulcers, penile discharge, or arthralgia. There had been no recent changes to his prescription medications, which included finasteride, terazosin, omeprazole, and an albuterol inhaler. He denied taking over-the-counter medications or supplements. The physical exam at the urgent care had revealed multiple blotchy, dark, 0.5- to 1-cm nonpruritic lesions that were desquamating. No oral lesions were seen. He had been given a diagnosis of hand-foot-mouth disease (HFMD) and reassured that it would resolve on its own in about 10 days. What's the diagnosis?