**Core Concept**
Posterior urethral valves (PUV) are a congenital anomaly leading to obstruction of the urethra, primarily affecting male infants. This condition requires prompt diagnosis to avoid long-term renal damage.

**Why the Correct Answer is Right**
The investigation of choice for diagnosing PUV is a _voiding cystourethrogram (VCUG)_. This imaging study involves injecting a contrast agent into the bladder, followed by a voiding cystourethrogram, which allows visualization of the urethra and bladder during urination. A characteristic "keyhole" sign or "mushroom" sign is often seen, indicating the presence of PUV.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is not the primary imaging modality for diagnosing PUV, although it may be used as an initial screening tool to assess renal function and detect any hydronephrosis.

**Option B:** A plain X-ray of the abdomen may show signs of upper urinary tract obstruction, such as hydronephrosis, but it is not specific for diagnosing PUV.

**Option C:** A CT urogram or MRI urogram may be used in some cases to evaluate the upper urinary tract, but they are not the first-line imaging modalities for diagnosing PUV.

**Clinical Pearl / High-Yield Fact**
The diagnosis of PUV often involves a combination of clinical presentation, imaging studies, and urodynamic testing. It is essential to recognize the classic signs and symptoms of PUV, such as urinary retention, urinary tract infections, or failure to thrive, to prompt early investigation and treatment.

**Correct Answer: D. Voiding cystourethrogram (VCUG)**