Kidney Stones (Nephrolithiasis)

Nephrolithiasis begins when urine becomes supersaturated with substances such as calcium, oxalate, uric acid, or cystine. Crystals then nucleate, enlarge, and aggregate within the renal tubules or collecting system. A small stone may remain in the kidney without symptoms until it enters the ureter. Renal colic is produced mainly by ureteral obstruction and muscular spasm, not simply by the stone rubbing against tissue. As urine accumulates above the obstruction, pressure rises and the ureter contracts forcefully around the stone. This produces severe pain that waxes and wanes rather than remaining steadily mild. Persistent obstruction can cause hydronephrosis, reduce filtration, and contribute to acute kidney injury. The most dangerous combination is obstruction plus infection. Bacteria trapped behind a blocked urinary tract can lead to sepsis while the obstruction prevents adequate drainage. Pain severity alone does not determine risk; fever, falling urine output, deteriorating renal function, and systemic illness change the priority to urgent source control.