The stubborn focal eruption on this 6-year-old boy’s left fourth finger remains unaffected after a one-week course of oral cephalexin, so his pediatrician refers him to dermatology. The problem initially manifested as a cluster of tiny blisters, which tingled a bit but didn’t hurt. When redness developed around it, the patient’s mother brought him for evaluation.
The boy experienced the same problem in the same location almost exactly a year ago. It was treated the same way and within a week or two had resolved.
The child is otherwise reasonably healthy, despite being allergy-prone. He has a history of seasonal allergies and is susceptible to skin infections, such as staph.
EXAMINATION
On the dorsal aspect of his finger are grouped vesicles on an erythematous base, measuring about 1 cm altogether. Several of these demonstrate central umbilication, and several are filled with pus. The erythema is minimal, and there is no tenderness on palpation. Palpation of the epitrochlear area reveals a tiny, nontender node.
What is the diagnosis?