Stones less than how many millimeters have about 90% chance of passing spontaneously?

• A. 2 mm
• B. 4 mm
• C. 6 mm
• D. 8 mm

Answer: (B)

Stone size is the main predictor of whether a ureteral stone will pass on its own. The smaller the stone, the higher the chance of spontaneous passage. The commonly cited figure is that stones smaller than about 4 millimeters have roughly a 90% chance of passing without intervention. As stones get larger, the likelihood of spontaneous passage drops, so 6–8 mm stones are far less likely to pass and often require medical or surgical management. Therefore, the threshold for about a 90% spontaneous passage is about 4 mm.