Which crystal is characteristically seen in acidic urine and suggests a cystinuria disorder?

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The main idea here is that certain urinary crystals act as clues to specific disorders. Cystine crystals are distinctive and strongly point to cystinuria. In acidic urine, cystine tends to precipitate because its solubility is low at lower pH. This leads to colorless, hexagonal (six-sided) crystals in the urine sediment. The presence of these hexagonal crystals is a classic sign of cystinuria, a genetic defect in renal reabsorption of the dibasic amino acids, including cystine, which results in high urinary excretion and stone formation.

Other crystals listed don’t fit as well. Calcium oxalate crystals are common and can appear in many urine conditions, not specifically tied to cystinuria. Amorphous urates can be seen in acidic urine but aren’t diagnostic for a particular disorder. Bilirubin crystals relate to liver or biliary problems rather than a defect in amino acid reabsorption. Thus, the hexagonal, colorless cystine crystals in acidic urine best indicate cystinuria.