For years, this 33-year-old woman has had a firm, pea-sized nodule on her left upper back. It was never a problem until recently, when it suddenly enlarged and became red, swollen, and tender.
She was prescribed antibiotics (trimethoprim and sulfa) by a provider at her local urgent care center. Dubious of the diagnosis—carbuncle—she sought referral to dermatology.
The patient claims to be in otherwise excellent health, with no history of similar problems. She denies manual manipulation of the lesion.
The patient is afebrile and in no distress. On her upper left back is a round, cystic lesion measuring 3.5 cm. It appears swollen and red. The erythema, though impressive, is confined to
the area immediately around the margins. Palpation reveals increased warmth and modest tenderness. A central punctum can be seen in the center of the fluctuant lesion.
After a brief discussion of options, the lesion is incised and drained under sterile conditions with lidocaine and epinephrine. Cheesy, odoriferous material is expressed, effectively flattening the lesion.
What is the diagnosis?