Severe Palmoplantar Keratoderma—Quiz Case

A 10-month-old boy presented with a slowly enlarging lesion on his left second toe; the lesion had been present since he was 6 months old. He was an otherwise healthy, well-developed infant who was the product of a fullterm, vaginal delivery. The lesion was asymptomatic and did not interfere with mobilization of the digit. A physical examination revealed a 4 5-mm, smooth, firm, pink papule over the dorsolateral aspect of the distal interphalangeal joint with a similar, smaller papule noted over the proximal nail fold (Figure 1). Findings from a full-body skin examination were otherwise normal. Radiographic examination showed no bony involvement. Owing to its continued growth and concern that it might interfere with his walking, an excisional biopsy was performed. Histologic examination of the excised specimen stained with hematoxylin-eosin (Figure2) and Masson trichrome (Figure 3) revealed the following. What is your diagnosis?