Situation: A 19-year-old man is admitted with dark, cola-colored urine, puffy eyelids on waking, and a blood pressure of 158/98 mmHg. Three weeks ago he had infected sores on his legs that healed without treatment. His anti-deoxyribonuclease B (anti-DNase B) titer is high and his serum complement (C3) is low. Acute post-streptococcal glomerulonephritis (PSGN) is diagnosed. He asks how skin sores that have already healed could damage his kidneys. Which explanation is accurate?

PSGN is an immune reaction, not a kidney infection. Weeks after a group A streptococcal skin or throat infection, antigen–antibody complexes deposit in the glomeruli and cause inflammation, which lowers complement (C3). Reduced filtration leads to sodium and water retention, edema, and hypertension.

Core mechanism: immune-complex deposition, not direct infection

The key to understanding this question is recognizing that acute post-streptococcal glomerulonephritis (PSGN) is a delayed immune-mediated injury, not an active bacterial infection of the kidney. The streptococci that caused the skin sores are already gone or no longer the direct problem by the time kidney symptoms appear. Instead, the immune response the body mounted against those bacteria becomes the source of renal damage ^[1][2].

During the original skin infection, the immune system produced antibodies against streptococcal antigens. These antibodies bind to bacterial antigens to form circulating antigen–antibody immune complexes. In PSGN, these complexes travel through the bloodstream and become trapped in the glomerular basement membrane and mesangium. Once deposited, they activate the complement cascade and trigger local inflammation, which damages the glomerular filtration barrier ^[1][2].

This explains the laboratory pattern in the scenario. The high anti-DNase B titer confirms a recent streptococcal infection, while the low C3 reflects complement consumption at the site of glomerular immune-complex deposition. Complement proteins are used up as the inflammatory reaction unfolds, so serum C3 falls ^[2][3]. The cola-colored urine results from glomerular inflammation allowing red blood cells to leak into the filtrate, and the edema and hypertension arise from sodium and water retention due to reduced glomerular filtration.

Watch out! The bacteria do not need to be alive in the kidney for PSGN to occur. The injury is caused by the host’s own immune response, which is why antibiotics are not the primary treatment once nephritis has developed ^[2][3].

Key point! The latent period of 1–3 weeks between the skin infection and the onset of dark urine, edema, and hypertension is a hallmark of immune-complex disease. A direct bacterial infection of the kidney would present much sooner and with different features such as fever, flank pain, and pyuria ^[2][3].

Option	Mechanism	Why it is incorrect or correct
1. Immune complexes lodged in glomeruli	Antigen–antibody complexes deposit in glomeruli, activate complement, and cause inflammation	Correct. This is the defining mechanism of PSGN [1][2]
2. Dehydration reduced renal blood flow	Prerenal hypoperfusion from volume loss	Incorrect. The scenario shows fluid retention with edema and hypertension, not dehydration
3. Bacteria traveled to the kidneys	Hematogenous bacterial seeding causing pyelonephritis or renal abscess	Incorrect. PSGN is not an active infection; bacteria are not multiplying in the kidney [2]
4. Bacterial toxins poisoned tubules	Direct tubular toxicity from streptococcal exotoxins	Incorrect. The injury is glomerular and immune-mediated, not tubular and toxin-mediated [1][2]

The distinction between immune-complex glomerular injury and direct bacterial infection also guides nursing assessment and monitoring. In PSGN, the priority is managing fluid overload, hypertension, and potential complications such as hypertensive encephalopathy or pulmonary edema. Urine output, daily weight, blood pressure, and serum creatinine are tracked closely, while antibiotics are reserved for any persistent streptococcal infection rather than for the nephritis itself ^[2][3]. The low C3 and high anti-DNase B titer are diagnostic clues that point to the immune mechanism, reinforcing that the healed skin sores set off a systemic immune response whose target became the glomerulus ^[1][2][3].