This is a lesion which starts as a round, erythematous macular lesion which is painless. This then becomes a haemorrhagic vesicle or blister, and evolves into a pustule with necrotic ulceration (a red halo around a black scab). It can be one lesion, but is sometimes multiple lesions.
It is commonly associated with pseudomonas aeruginosa infection and often occurs in immunocompromised patients.
Diagnosis is from gram stain of the fluid in the pustule, or skin biopsy.
Treatment will be with antibiotics. Surgery is sometimes required with extensive necrosis.