**Core Concept**
Vesicovaginal fistula (VVF) is an abnormal communication between the bladder and vagina, leading to continuous urinary leakage. Accurate diagnosis and assessment of VVF require a comprehensive evaluation, including imaging and urodynamic studies.

**Why the Correct Answer is Right**
The most useful investigation for VVF is cystoscopy with hydrodistension under anesthesia. This procedure involves filling the bladder with water while examining the bladder and vaginal walls using a cystoscope. Hydrodistension helps to visualize the fistula and assess its size and location. Additionally, urodynamic studies, such as voiding cystourethrography (VCUG), may be performed to evaluate the bladder's function and capacity.

**Why Each Wrong Option is Incorrect**
**Option A:** Urinary tract ultrasound is not sufficient for diagnosing VVF, as it may not detect small fistulas or those located near the bladder neck.

**Option B:** Voiding cystourethrogram (VCUG) is a useful investigation for diagnosing VVF, but it is not the most useful investigation, as it may not provide detailed information about the fistula's location and size.

**Option C:** Intravenous urography (IVU) is not a primary investigation for VVF, as it may not provide sufficient detail about the fistula's location and may not be sensitive enough to detect small fistulas.

**Clinical Pearl / High-Yield Fact**
Cystoscopy with hydrodistension is a crucial diagnostic tool for VVF, and it should be performed under anesthesia to ensure accurate visualization of the fistula.

**Correct Answer: C. Voiding cystourethrography (VCUG)**