This painful black eschar with slight surrounding angulated purpura in a patient on dialysis is consistent with calciphylaxis. Calciphylaxis is an uncommon syndrome that typically occurs in patients with advanced kidney dysfunction and an elevated calcium-phosphorus product (>60-70 mg²/dL²), which may be present in this patient who has difficulty with medication adherence. Calciphylaxis results from abnormal deposition of calcium within the lumen of the arterial vasculature, compromising blood flow with distal ischemia resulting in painful tissue necrosis. It has a poor prognosis with a 60% to 80% 1-year mortality rate, with patients often succumbing to infectious complications. Management requires pain control, meticulous skin care, and surveillance for associated infection due to increased risk of tissue ischemia. Directed therapy is preferably multi-interventional and typically includes aggressive lowering of the calcium-phosphorus product (by dialysis and non-calcium phosphate binders) to less than 55 mg²/dL², administration of sodium thiosulfate (mechanism unclear), and lowering of the parathyroid hormone level (by surgery or cinacalcet). Hyperbaric oxygen, careful debridement, and bisphosphonates may also be helpful.

Vasculitis is inflammation of the blood vessels and can affect any size vessels. The most common cutaneous vasculitis is a small vessel vasculitis, which manifests as palpable purpura (nonblanching red papules). Medium vessel vasculitis (such as polyarteritis or granulomatosis with polyangiitis) can lead to ulcers in the skin, but the lesions are often strikingly inflamed with an active erythematous-to-red border and often surrounding livedo-like changes. A single, thick black eschar, as seen in this patient, would be uncommon.

Spider bites are exceedingly rare and overdiagnosed in clinical practice. Bites that do occur usually follow an exposure to secluded, dark, neglected areas such as a woodshed. Many patients living in environments rich in brown recluse spiders never receive a bite, and most patients diagnosed with bites have alternative diagnoses. Additionally, only a small percentage of brown recluse spider bites become necrotic. In those that do, the bite is usually exquisitely painful, rapidly progressing with deep tissue necrosis and accompanying systemic symptoms. Most necrotic bites are smaller than the lesion seen in this patient and usually stop expanding and start healing within about 10 days.

Pyoderma gangrenosum is a neutrophilic dermatosis in which the host's neutrophils cause autoinflammation in the skin, leading to painful, exudative, rapidly progressive ulcers. The ulcers typically start as a pustule or shallow erosion in response to minor trauma and rapidly expand. Classic PG ulcers will have a rim of violaceous erythema and edema; an overhanging lip; thin, anastomosing strands of intact epidermis hanging over the ulcer base; and excessive serous exudates that can lead to fibrinous “slough” over the ulcer base. This patient's skin findings are not consistent with pyoderma gangrenosum.