Prednisone is appropriate for this patient. His findings demonstrate the presence of a small-vessel vasculitis affecting the skin, joints, kidneys, and gastrointestinal tract. Deposition of IgA in the skin confirms the diagnosis of adult-onset Henoch-Schönlein purpura (HSP). In children, HSP is generally a benign, self-limited condition, and treatment is most commonly supportive pending spontaneous remission. HSP in adults is less common and typically is more severe. Although adult HSP also tends to run a self-limited course, adults with HSP are more likely to experience severe disease and to accumulate irreversible organ damage before the acute disease resolves. In this case, the involvement of multiple organ systems, including the gastrointestinal tract, probably warrants prednisone treatment based upon expert consensus recommendation.

Cyclophosphamide is an alkylating agent and potent immunosuppressant. It is commonly used for treatment of severe autoimmune disease such as systemic lupus erythematosus and ANCA-associated vasculitis. Its use in adult-onset HSP is less well established; although it is sometimes used in conjunction with prednisone for severe HSP nephritis, it would not be a first-choice therapy in the absence of prednisone use.

Dapsone is an antibiotic that has antileukocyte activity and is occasionally used to treat the leukocytoclastic vasculitides, including HSP. However, given the severity of this patient's condition, dapsone is less likely to be effective, and prednisone use is warranted.

Like all NSAIDs, ibuprofen may help alleviate joint pain and swelling and may have some modest effect on reducing the inflammation of small-vessel vasculitis. However, ibuprofen is unlikely to be adequately effective in a serious case such as this. Moreover, the nephrotoxic and antiplatelet effects of an NSAID would be undesirable in this patient who already has acute kidney injury and intestinal bleeding.