Kidney biopsy is the most appropriate next step in management. This patient has worsening kidney function and new hypertension associated with significant abnormalities on urinalysis, including proteinuria, leukocytes indicating inflammation, hematuria with dysmorphic erythrocytes (acanthocytes), and erythrocyte casts on microscopy. These findings strongly suggest glomerulonephritis. Constitutional symptoms such as fever and arthralgia, as well as nonblanching rash (palpable purpura) and weight loss, further suggest vasculitis as the most likely etiology. Urgent serologic evaluation and kidney biopsy are indicated to diagnose the cause of glomerulonephritis and guide management.

Although this patient has a history of urinary tract infection (UTI), reports fever, and has leukocytes on her urinalysis, her specific lack of symptoms of UTI or other findings consistent with this diagnosis (no costovertebral angle tenderness or bacteria on microscopic urinalysis) make this a less likely diagnosis. UTI would also not be expected to lead to development of hypertension or kidney failure. Therefore, antibiotics are not indicated.

Cystoscopy is typically used to evaluate for hematuria due to a bladder source, such as bladder cancer. The hematuria associated with a bladder source of bleeding is usually structurally normal compared with the dysmorphic erythrocytes seen in this patient; acanthocytes suggest a glomerular source of bleeding. Also, a bladder source of bleeding, such as a malignancy, would not explain the other findings seen on her urinalysis (significant proteinuria, leukocytosis) or her new hypertension and kidney failure, making cystoscopy an inappropriate next step in management.

Kidney ultrasonography can identify structural abnormalities, including nephrolithiasis. However, in this patient with a clinical picture consistent with acute glomerulonephritis, kidney imaging would not be of high diagnostic yield.