The most appropriate management for this patient is to continue current therapy. This patient with methicillin-resistant Staphylococcus aureus endocarditis is found to have worsening kidney function since hospitalization. The differential diagnosis includes infection-related glomerulonephritis (IRGN), drug-induced nephrotoxicity, acute interstitial nephritis (AIN), and septic emboli. The finding of a nephritic urine sediment (erythrocytes, erythrocyte casts, and proteinuria) in an azotemic patient with an active infection suggests IRGN. IRGN is an immune complex–mediated disease most frequently associated with nonstreptococcal infections, with the antigen in the immune complex derived from the infectious agent. Immune complexes deposit in the subepithelial area and activate complement with recruitment of inflammatory cells, leading to a proliferative GN. The likelihood of IRGN is high in this patient given the low C3 complement, the absence of cryoglobulins, and the lack of clinical findings suggestive of other causes on the differential diagnosis.

Glucocorticoids are not typically used in IRGN because there is usually improvement with control of the associated infection.

A kidney biopsy is not indicated because the probability of IRGN is high. However, biopsy would be appropriate if this patient's kidney function fails to improve with treatment of the underlying infection.

Drug-induced tubular toxicity (for example, with vancomycin) typically occurs after 7 to 10 days of antibiotic therapy and the urine sediment does not show cells, unlike in this patient. Antibiotic-induced AIN is typically associated with mild proteinuria, erythrocytes, leukocytes, and leukocyte casts on urinalysis. Eosinophiluria, recurrence of fevers, rash, and peripheral eosinophilia may also be seen and typically occur after 7 to 10 days of therapy, none of which is present in this patient. Therefore, switching vancomycin to daptomycin is not appropriate.