This patient likely has infection-related glomerulonephritis (IRGN) following a streptococcal infection. Supportive evidence includes preceding symptoms of an upper respiratory tract infection suggestive of streptococci (rapid streptococcal antigen test is positive and antistreptolysin O antibodies are elevated), followed by the nephritic syndrome in 1 week, and low C3 levels with normal C4 levels (suggesting an alternative pathway of complement activation, which is typical of IRGN). Most patients will show spontaneous resolution of nephritis with conservative management (antibiotics, blood pressure management, and diuretics).

IgA nephropathy (IgAN) is the most common form of glomerulonephritis. Asymptomatic microscopic hematuria with or without proteinuria is the most common presentation of IgAN, and episodic gross hematuria following an upper respiratory tract infection is a classic presentation. Kidney manifestations usually occur concomitantly with the respiratory infection in IgAN (“synpharyngitic” nephritis), as opposed to the typical 7- to 10-day latent period with IRGN. Moreover, complement levels are typically normal in IgAN, whereas C3 is typically low and C4 is normal in IRGN.

Lupus nephritis may occasionally be precipitated by infections. Patients with lupus typically experience systemic manifestations such as rash and arthritis, although kidney-limited disease is sometimes seen. Both C3 and C4 complement levels are depressed in this condition due to the classical pathway of complement being activated. In IRGN, C3 is typically depleted, with normal levels of C4 due to activation of the alternative pathway of complement.

Small-vessel vasculitis is also associated with glomerulonephritis. However, there are frequently other clinical findings of vasculitis present, and complement levels are typically normal.