## Pathophysiology of Calcium Oxalate Stone Formation **Key Point:** Hyperoxaluria does NOT reduce urine pH — it increases stone risk by increasing the concentration of oxalate ions, which directly promotes calcium oxalate crystal nucleation and precipitation. The statement confuses the mechanism. ### Correct Metabolic Risk Factors | Risk Factor | Mechanism | Clinical Significance | |---|---|---| | **Hypercalciuria** | Elevated urinary calcium (>200 mg/day in women, >250 mg/day in men) | Most common metabolic abnormality in stone formers | | **Hyperoxaluria** | Elevated urinary oxalate (>40 mg/day) | Increases calcium oxalate saturation; does NOT lower pH | | **Hypocitraturia** | Low urinary citrate (<320 mg/day) | Citrate is a stone inhibitor; deficiency increases risk | | **Low urine pH** | Acidic urine (pH <5.5) | Promotes uric acid and calcium oxalate precipitation | | **Low urine volume** | Dehydration, inadequate fluid intake | Increases solute concentration and crystal formation | | **Hyperuricemia** | Elevated serum/urinary uric acid | Associated with gout; uric acid crystals act as nidus for calcium oxalate | **High-Yield:** Hyperoxaluria increases stone risk by **increasing solute concentration and saturation**, NOT by lowering pH. Urine pH is primarily affected by acid-base status and diet (animal protein lowers pH; citrus and vegetables raise pH). ### Why Hypocitraturia Is Protective When Present **Clinical Pearl:** Citrate is a natural stone inhibitor — it chelates calcium and reduces calcium oxalate saturation. Low urinary citrate (hypocitraturia) is therefore a RISK factor, not protective. The question stem correctly identifies this as a risk factor. ### Association with Gout and Hyperuricemia Hyperuricemia and gout are well-established risk factors for calcium oxalate stones. Uric acid crystals serve as a nidus for heterogeneous nucleation of calcium oxalate, increasing stone formation risk.
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