Kidney stones
- Lesion
Supersaturated urine lets salts crystallize into stones that obstruct, bleed and infect; calcium stones are most common.
What it is
Stones form when urine becomes supersaturated with a salt, when inhibitors of crystallization (citrate, pyrophosphate, uromodulin) are low, or when urine pH favours precipitation. Most are calcium oxalate, often with hypercalciuria; struvite stones form around urease-producing bacteria in alkaline urine and can grow into staghorn casts of the pelvis; uric acid stones form in acid urine with high urate; cystine stones come from an inherited transport defect. Stones are usually unilateral and form in the calyces and pelvis.
Structure
- Stones range from sand to large branched casts of the pelvis and calyces (staghorn).
- Small stones pass into the ureter and lodge at narrow points: the ureteropelvic junction, the pelvic brim and the vesicoureteral junction.
- The kidney may show hydronephrosis, pyelonephritis and scarring.
Kidney stones
| Stone | Setting | Urine pH | Clue |
|---|---|---|---|
| Calcium oxalate or phosphate | Idiopathic hypercalciuria, hyperparathyroidism, enteric hyperoxaluria | Oxalate: any; phosphate: alkaline | Radiopaque; envelope or dumbbell crystals |
| Struvite | Infection with urease-producing bacteria (Proteus) | Alkaline | Staghorn stones, recurrent infection; coffin-lid crystals |
| Uric acid | Gout, tumour lysis, acid urine, metabolic syndrome | Acid | Radiolucent on plain films; rhomboid crystals |
| Cystine | Cystinuria (inherited) | Acid | Hexagonal crystals, stones from childhood or youth |
Clinical notes
- Renal colic radiating from flank to groin, with hematuria; obstruction plus infection is an emergency.
- Recurrent stones call for a metabolic study of urine and blood, including calcium and parathyroid hormone.
Test yourself
Think of the answer, then tap the question to see it.
Which stone forms in alkaline urine infected by Proteus?
Struvite (magnesium ammonium phosphate), often a staghorn stone.
Which stones are radiolucent on plain radiographs?
Uric acid stones.
What is the most common metabolic abnormality in calcium stone formers?
Hypercalciuria, usually without hypercalcemia.
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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Renal Calculi, Nephrolithiasis (StatPearls (NCBI Bookshelf), 2024); Cystinuria: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.