A 42 year old man presented with severe pain in the lower back radiating to the pubic symphysis. Ultrasonography revealed a stone in the left ureter, measuring 3mm in size. The stone is most likely to be lodged in which of the following sites?
**Core Concept**
The ureter is a narrow muscular tube that carries urine from the kidneys to the bladder. A stone in the ureter can cause severe pain due to the obstruction of urine flow. The location of the stone within the ureter is crucial in determining the severity of symptoms and the likelihood of successful passage.
**Why the Correct Answer is Right**
A 3mm stone in the ureter is likely to be lodged at the ureteropelvic junction (UPJ) or the ureterovesical junction (UVJ). However, given the size of the stone, it is more likely to be lodged at the UPJ. This is because the UPJ is a narrow region where the ureter emerges from the renal pelvis and is surrounded by a thick layer of connective tissue. The smaller diameter of this region makes it more prone to stone impaction.
At the UPJ, the stone will cause obstruction of urine flow, leading to severe pain in the lower back radiating to the pubic symphysis. The pain is often described as colicky in nature and is exacerbated by movement.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the stone is unlikely to be lodged at the renal pelvis, given its small size and the fact that it is causing severe pain in the lower back radiating to the pubic symphysis.
**Option B:** This option is incorrect because the stone is unlikely to be lodged at the ureterovesical junction (UVJ), given its small size and the fact that it is causing severe pain in the lower back radiating to the pubic symphysis.
**Option D:** This option is incorrect because the stone is unlikely to be lodged at the distal ureter, given its small size and the fact that it is causing severe pain in the lower back radiating to the pubic symphysis.
**Clinical Pearl / High-Yield Fact**
A stone in the UPJ is a classic cause of colicky pain in the lower back radiating to the pubic symphysis. It is essential to identify the location of the stone to determine the best course of treatment.
**Correct Answer:** C.