Post Streptococcal Erythema Nodosum
Erythema (red) Nodosum (painful) is an inflammation of the subcutaneous fat, and is triggered by a variety of circumstances, but most of them involve immune complexes being deposited in the medium sized vessels of the septae of the panniculus.
The appearance is of a slightly elevated erythematous plaque often with, as you see above, a "vascular steal" phenomenon. They are, as the name implies, very tender. The location as well as the multiplicity and the tenderness all are clues as to its identity.
Most cases occur in young adult females, are usually multiple, usually pretibial and can be associated with other symptomatology associated with immune complexes such as joint pain, fever, headache and malaise. Ulceration does not occur and they heal without scarring.
Historically, tuberculosis was the most common associated disease, but currently we see it most commonly in association with streptococcal disease, particularly pharyngitis. Of course, it can also be associated with gut pathogens such as Yersinia, Salmonella and Shigella; but also a kazillion other infectious diseases (too numerous to be listed here) and also, of course, Sarcoidosis. However, for two additional points, name two deep fungal diseases that cause EN. Also, the estrogens in oral contraceptives can trigger this condition, as can other meds such as sulfonamides.