The diagnosis is Staphylococcus infection–associated glomerulonephritis (SAGN), as indicated by methicillin-sensitive Staphylococcus aureus (MSSA) in blood and synovial fluid, acute kidney injury (AKI) with macroscopic hematuria, nephrotic proteinuria, C3 staining, glomerular hypercellularity in renal pathology, and exclusion of other possible etiologic causes of AKI via screening tests. No single clinical or pathological feature is pathognomonic for SAGN. Clinical features, biopsy findings, culture results, and urinalysis findings should be taken into consideration at the time of diagnosis.