A 52-year-old man from Delhi presents to the emergency department with acute left flank pain and haematuria. Non-contrast CT abdomen shows a 8 mm stone in the left ureter at the level of the iliac vessels. On examination, he has tenderness in the left costovertebral angle. The urologist explains that the stone is lodged at one of the three physiological constriction sites of the ureter. Which of the following anatomical locations represents the MOST common site of ureteric stone impaction?

Explanation

## Anatomical Constriction Sites of the Ureter The ureter has three physiological constriction sites where stones are most likely to lodge: | Site | Location | Frequency of Stone Impaction | Anatomical Basis | |------|----------|------------------------------|------------------| | **First** | Ureteropelvic junction (UPJ) | ~35–40% | Transition from renal pelvis to ureter; change in calibre | | **Second** | Pelvic brim (crossing iliac vessels) | ~30–35% | Ureter crosses iliac bifurcation; external compression | | **Third (Most common / narrowest)** | Ureterovesical junction (UVJ) | ~40–50% | Entry into bladder; intramural course is the narrowest segment | **Key Point:** The **ureterovesical junction (UVJ)** is the narrowest and **most common** site of ureteric stone impaction overall. The intramural portion of the ureter as it traverses the bladder wall is only ~1–2 mm in diameter, making it the tightest physiological constriction. Most ureteric stones (up to 50% in some series) lodge here. **Clinical Pearl:** Although the CT in this vignette describes the stone "at the level of the iliac vessels" (pelvic brim — the second constriction site), the question asks for the **MOST common** site of ureteric stone impaction in general, which is the **UVJ**. The UVJ is the narrowest segment of the entire ureter and is the site where stones most frequently become impacted and require intervention (ureteroscopy ± laser lithotripsy). **High-Yield:** The three constriction sites in order of anatomical narrowing: 1. **UPJ** — transition from renal pelvis; ~2–3 mm 2. **Pelvic brim** — crossing iliac vessels; external compression 3. **UVJ** — intramural course; narrowest (~1–2 mm); **most common impaction site** **Mnemonic:** **PIP** — **P**yeloureteric junction, **I**liac crossing, **P**elvic-ureteric junction (UVJ) — with the last "P" being the most problematic for stones. ## Why This Matters Clinically Stones at the UVJ cause severe colicky pain radiating to the groin/scrotum/labia and are associated with urinary urgency and frequency due to bladder trigone irritation. They are the most common indication for ureteroscopic stone removal. Understanding the three constriction sites is essential for predicting where stones lodge and planning management (conservative vs. ureteroscopy vs. ESWL) [Campbell-Walsh Urology, 12th ed., Ch. 52; Smith's General Urology, 17th ed.].