**Core Concept**
Urinary ascites in a neonate is a rare condition characterized by the accumulation of urine in the peritoneal cavity, often due to an obstruction in the urinary tract. The most common cause is related to an anatomical abnormality that affects the flow of urine.

**Why the Correct Answer is Right**
Posterior urethral valves (PUV) are congenital obstructive lesions that occur in male infants, typically between 1-3 months of age. The presence of these valves causes a significant obstruction to the flow of urine from the bladder, leading to a backup of urine into the kidneys and eventually into the peritoneal cavity. This condition is often associated with symptoms such as urinary tract infections, renal impairment, and in severe cases, urinary ascites. The valves are usually diagnosed on imaging studies such as ultrasound, which shows the characteristic "keyhole" appearance of the urethra.

**Why Each Wrong Option is Incorrect**
**Option A:** Bilateral Pelvic Ureteric Junction (PUJ) obstruction is a condition that can cause hydronephrosis and renal impairment, but it is not typically associated with urinary ascites. The obstruction occurs at the junction between the renal pelvis and the ureter, which can lead to a buildup of urine in the kidneys, but it does not directly cause ascites.
**Option B:** Infant polycystic kidney disease is a genetic disorder that leads to the formation of multiple cysts in the kidneys, causing renal enlargement and impairment. While it can cause urinary tract infections and kidney damage, it is not typically associated with urinary ascites.
**Option D:** Meatal stenosis is a narrowing of the urethral meatus, which can cause urinary tract infections and difficulty urinating, but it is not typically associated with urinary ascites.

**Clinical Pearl / High-Yield Fact**
Posterior urethral valves are a classic example of a congenital anomaly that can cause significant morbidity if left untreated. Prompt diagnosis and treatment are essential to prevent long-term renal damage and other complications.

**✓ Correct Answer: C. Posterior urethral valve**