Which statement best describes the initiation of kidney stone formation?

• A. Calcium and oxalate never precipitate in urine
• B. Supersaturation of calcium and oxalate in urine leads to crystals and stones
• C. Stone formation is always due to infection
• D. Calcium oxalate is always radiopaque

Answer: (B)

Initiation of kidney stone formation hinges on urine becoming supersaturated with stone-forming substances. When calcium and oxalate reach high enough concentrations, they can precipitate out of solution and form crystals; these crystals can grow and aggregate, eventually developing into a stone if they aren’t sufficiently inhibited by factors like citrate or if urine flow is inadequate. This is why the statement about supersaturation leading to crystals and stones best captures how stones start. While infections can contribute to some stones (such as struvite stones), most stones form without infection. And although calcium-containing stones are typically visible on radiographs, saying calcium oxalate is always radiopaque is too absolute— imaging can vary and some stones may be radiolucent under certain conditions, so that claim isn’t universally accurate.