**Core Concept**
The question is testing the knowledge of congenital anomalies of the upper urogenital tract, specifically the most common anomaly. This concept is essential in pediatrics, as early diagnosis and management are critical to prevent long-term complications.

**Why the Correct Answer is Right**
Uretero-pelvic junction (UPJ) stenosis is the most common congenital anomaly of the upper urogenital tract. It occurs due to abnormal development of the ureteropelvic junction, leading to obstruction of the ureter. This obstruction can cause hydronephrosis, renal scarring, and chronic kidney disease if left untreated. The exact pathophysiology involves the abnormal formation of the ureteropelvic junction, which disrupts the normal flow of urine from the kidney to the bladder.

**Why Each Wrong Option is Incorrect**
**Option B:** Ectopic urethral opening is a congenital anomaly of the lower urogenital tract, not the upper urogenital tract. It involves the abnormal opening of the urethra on the perineum or in the vagina, leading to urinary incontinence and other complications.

**Option C:** Ureterocele is a congenital anomaly of the ureter, but it is less common than UPJ stenosis. It involves the dilation of the distal ureter, which can cause obstruction and hydronephrosis.

**Option D:** Ectopic ureter is a congenital anomaly where the ureter opens abnormally, either in the bladder or outside the urinary tract. While it is a significant anomaly, it is not the most common anomaly of the upper urogenital tract.

**Clinical Pearl / High-Yield Fact**
UPJ stenosis is often associated with other congenital anomalies, such as horseshoe kidney, and can be diagnosed prenatally using ultrasound. Early recognition and treatment are crucial to prevent long-term complications and preserve renal function.

**✓ Correct Answer: A. Uretero pelvic junction stenosis**