**Core Concept**
Posterior urethral valves (PUV) are a congenital anomaly that occurs in male infants and children, characterized by the presence of abnormal folds of tissue in the urethra that can cause obstruction to the flow of urine. This condition can lead to urinary retention, hydronephrosis, and other complications.

**Why the Correct Answer is Right**
The presence of PUVs in male children is the most common cause of urinary retention due to the obstructive nature of the abnormal folds of tissue. This obstruction can cause the bladder to become distended, leading to urinary retention. The exact mechanism involves the abnormal folds of tissue causing an obstruction to the normal flow of urine, which can be exacerbated by the narrow diameter of the male urethra. PUVs are typically diagnosed in infancy or early childhood, often after a prenatal ultrasound or during a routine physical examination.

**Why Each Wrong Option is Incorrect**
**Option B:** Urethral stenosis refers to a narrowing of the urethra, which can indeed cause urinary retention. However, it is not the most common cause in children. Urethral stenosis can be caused by various factors, including injury or surgical complications.
**Option C:** Urethral aplasia is a rare congenital anomaly where the urethra is either absent or severely underdeveloped. While it can cause urinary retention, it is much less common than PUVs.
**Option D:** Ectopic ureteric opening refers to an abnormal opening of the ureter that can cause urinary incontinence or retention. However, it is not the most common cause of urinary retention in children.

**Clinical Pearl / High-Yield Fact**
Posterior urethral valves are a classic example of an obstructive uropathy that can lead to significant morbidity if left untreated. Early diagnosis and treatment are crucial to prevent long-term complications such as renal scarring and hypertension.

**✓ Correct Answer: A. Posterior urethral valves**