The correct answer is tuberculosis. The diagram shows an undermined (overhanging) edge, where the skin edge projects over a wider cavity so the base is broader than the surface opening. This happens when subcutaneous tissue is destroyed faster than the overlying skin, the classical behaviour of a tuberculous ulcer (and of an amoebic ulcer). Tuberculous ulcers are shallow with a pale wash-leather floor, thin bluish undermined edges and scanty seropurulent discharge, often over the neck from a collar-stud abscess of cervical nodes, caused by Mycobacterium tuberculosis. Biopsy shows caseating epithelioid granulomas; treatment is antitubercular therapy.

Syphilitic gummatous ulcers have a punched-out vertical edge. Squamous cell carcinoma gives an everted, rolled-out edge as proliferating tumour heaps over normal skin. Basal cell carcinoma (rodent ulcer) gives a rolled-in, beaded, pearly edge with telangiectasia.

Key point: Ulcer edges are a favourite one-liner: undermined = tuberculous, punched-out = syphilitic or trophic, everted = squamous carcinoma, beaded/rolled = basal cell carcinoma, sloping = healing.