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Renal calculi occur due to an imbalance of the factors that facilitate or prevent stone formation Overall, increased urinary concentrations of calcium (hypercalciuria), oxalate (hyperoxaluria), and uric acid (hyperuricosuria) promote salt crystallization, whereas increased urinary citrate and high fluid intake inhibit calculi formation

Normally, citrate excreted by the kidneys binds to ionized calcium in the urine, preventing the formation of insoluble calcium-oxalate
complexes. When urinary citrate is low (hypocitraturia), increased calcium availability leads to formation of calcium-oxalate complexes that can precipitate and form calcium oxalate stones. Hypocitraturia often occurs in the setting of chronic metabolic acidosis (eg, distal renal tubular acidosis, chronic diarrhea) due to enhanced renal citrate reabsorption Supplemental oral potassium citrate is often prescribed to prevent
recurrent calcium stones.

(Choice A) Individuals with higher (but not excessive) calcium intake paradoxically have a lower risk of calcium oxalate stone formation. Dietary calcium binds oxalate in the gut to form insoluble calcium oxalate, which is eliminated in the feces. This reduces the amount of oxalate absorbed into the body and excreted in the urine, reducing stone formation

(Choice B) In patients with inadequate dietary intake of potassium, increased tubular reabsorption of potassium leads to increased citrate reabsorption, which facilitates formation of calcium oxalate complexes in the renal tubules. Higher potassium intake promotes urinary excretion of citrate and lowers urinary calcium excretion, leading to a lower risk of calcium oxalate stones.

(Choice C) High fluid intake prevents supersaturation of urine with stone-forming ingredients. Low fluid intake increases the urinary concentration of these ions regardless of their absolute amounts, promoting stone formation

(Choice D) Excessive oxalate intake (eg, chocolate, spinach, rhubarb) leads to increased intestinal absorption of free oxalate, which is then excreted in the urine where it promotes formation of calcium oxalate stones. Intestinal malabsorption syndromes (eg, Crohn disease) can also cause hyperoxaluria because calcium becomes bound by unabsorbed lipids in the gut

Educational objective:

Renal calculi occur due to an imbalance of the factors that facilitate or inhibit stone formation. Increased urinary concentrations of calcium, oxalate, and uric acid promote salt crystallization, whereas increased urinary citrate concentration and high fluid intake prevent calculi formation
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