The ACE inhibitor captopril is the most appropriate treatment for this patient who most likely has scleroderma renal crisis (SRC) in the setting of diffuse cutaneous systemic sclerosis (DcSSc). SRC occurs in 10% to 15% of patients with systemic sclerosis and is more frequent in DcSSc compared with limited cutaneous systemic sclerosis. Vascular involvement of afferent arterioles leads to glomerular ischemia and hyperreninemia. The typical presentation is acute onset of oliguric kidney disease and severe hypertension, mild proteinuria, urinalysis with few cells or casts, microangiopathic hemolytic anemia, and thrombocytopenia. Some patients develop pulmonary edema and hypertensive encephalopathy. Normal blood pressure may be present in up to 10%. This patient presents acutely with a rapid rise in serum creatinine consistent with acute kidney injury, with a bland urinalysis and non–nephrotic-range proteinuria as well as neurologic symptoms suggestive of encephalopathy. Although her blood pressure is almost normal, these findings are highly suggestive of SRC. Treatment with an ACE inhibitor is essential to restore kidney function and manage hypertension associated with SRC. Captopril is the preferred ACE inhibitor because it has been the most extensively studied agent in this clinical setting, and its short half-life allows rapid titration.

Cyclophosphamide is a potent immunosuppressant used to treat severe or life-threatening manifestations of certain diseases such as systemic lupus erythematosus or systemic vasculitis. It is ineffective in treating SRC, which is vascular and noninflammatory.

This patient does not have inflammatory end-organ involvement; therefore, methylprednisolone is not needed. Glucocorticoids are not useful in SRC, and intravenous glucocorticoids may cause worsening symptoms.

Sildenafil can be used to treat pulmonary hypertension or finger ulcerations but is not appropriate for SRC, which is primarily mediated through the renin-angiotensin axis.