At birth, this child had a lesion on his shoulder that now—a year later—has doubled in size. His parents report no systemic symptoms or medication use for their son. They say that the child exhibits no distress; he does not attempt to scratch at the affected patch of skin. However, they observe that if the lesion is touched, it swells and then (within minutes) returns to normal.
There is no family history of similar problems. However, both the patient and his mother are highly atopic.
The lesion—a low, orange, oval plaque—measures about 3.5 x 2 cm. Barely palpable, it urticates when stroked with a fingernail edge but does not appear to cause any discomfort.
No other lesions of note are found. The child appears quite healthy and is in no distress.
What is the diagnosis?