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Renal stones vary in composition, radiodensity, and the urine conditions that favor their formation. Calcium stones are most common, while struvite stones are associated with urease-producing bacteria and may form staghorn calculi; cystine stones point to hereditary cystinuria. Obstruction can occur anywhere from the renal pelvis to the urethral meatus and may result from stones, prostatic enlargement, tumors, congenital narrowing, or other causes. Rising urinary pressure produces hydronephrosis and, over time, tubular atrophy, interstitial fibrosis, and nephron loss. Bilateral complete obstruction can cause rapid post-renal azotemia with reduced or absent urine output, whereas unilateral or partial obstruction may be clinically silent. Relief may allow functional recovery, but chronic obstruction can leave irreversible damage.