**Core Concept**
Vesico-vaginal fistula (VVF) is an abnormal communication between the bladder and vagina, leading to continuous urinary leakage. The investigation of choice for diagnosing VVF is crucial in planning the appropriate management.

**Why the Correct Answer is Right**
Methylenediamine Tetraacetic Acid (METHYLINE BLUE) instillation is the investigation of choice for VVF. This involves instilling a blue dye into the bladder through a catheter. The patient is then asked to void, and the presence of blue dye in the vagina confirms the presence of a fistula. This method is non-invasive and provides a clear diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Cystoscopy is a useful diagnostic tool for bladder pathology, but it may not be able to visualize the fistula in cases where the communication is small or the fistula is located in a difficult-to-reach area.

**Option B:** Urethrography involves injecting a contrast agent into the urethra to visualize the urinary tract. While it can be useful in diagnosing urethral injuries or strictures, it is not the primary investigation for VVF.

**Option C:** Voiding cystourethrogram (VCUG) involves injecting a contrast agent into the bladder and taking X-rays as the patient voids. While it can provide information on bladder and urethral pathology, it is not the best investigation for diagnosing VVF.

**Clinical Pearl / High-Yield Fact**
The key to diagnosing VVF is to look for a history of prolonged obstructed labor, which is the most common cause of VVF. This can be remembered using the mnemonic "OB - Oxytocin-induced obstruction, leading to Bladder damage and VVF".

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