History

A 38-year-old man presented with a 2-month history of painless left groin mass and multiple erythematous to violaceous nodules and plaques on both groins and thighs. The skin lesions had gradually increased in size and number. Past medical history was unremarkable. Physical examination showed a soft and non-tender mass in the left groin and multiple bean-sized erythematous nodules and plaques on both groins and thighs. There were neither any other systemic symptoms nor lymphadenopathy. Laboratory tests were normal.

Pelvic magnetic resonance imaging (MRI) (Fig. 1) and computed tomography (CT) of the chest and abdomen were performed. Histopathology was also examined (Fig. 2). Neither abnormal lymphadenopathy nor visceral involvement was noted on chest and abdominal CT scan. What is the diagnosis?

Fig. 1
figure 1

Pelvic MRI (including a axial TSE-T2 weighted, b coronal T2-weighted with fat saturation, and c axial fat-suppressed T1-weighted images after gadolinium administration) was followed by left groin subcutaneous tissue and skin core biopsy

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Fig. 2
figure 2

a Hematoxylin and eosin stain 10× and 40×, and b CD4 stain 10× and 40×

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