**Core Concept**
A ureterovaginal fistula is an abnormal communication between the ureter and the vagina, resulting from damage to the ureter during surgery, trauma, or other pelvic pathology. This condition disrupts the normal flow of urine from the kidneys to the bladder, leading to continuous urinary leakage through the vagina.

**Why the Correct Answer is Right**
The patient's symptoms, such as continuous urinary leakage despite occasional voiding, suggest an abnormal communication between the urinary tract and the vagina. A ureterovaginal fistula is a plausible explanation for this presentation, as urine from the ureter can drain directly into the vagina. This condition often occurs after surgeries like hysterectomy, which can damage the ureter. The ureter's normal function is to transport urine from the kidney to the bladder through peristalsis. In a ureterovaginal fistula, urine bypasses the bladder and flows directly into the vagina.

**Why Each Wrong Option is Incorrect**
**Option A:** Vesico-vaginal fistula typically presents with continuous urinary leakage, but the patient in this scenario occasionally voids urine, suggesting that the bladder is not completely bypassed by the fistula.

**Option C:** Stress incontinence typically occurs during physical stress, such as coughing, sneezing, or exercising, and is not associated with continuous urinary leakage.

**Option D:** Overflow incontinence occurs when the bladder is overdistended, leading to involuntary leakage of urine. However, the patient in this scenario has continuous urinary leakage, not just occasional leakage due to bladder overdistension.

**Clinical Pearl / High-Yield Fact**
A ureterovaginal fistula is a classic complication of pelvic surgery, particularly hysterectomy, and can be diagnosed using intravenous pyelography (IVP) or cystoscopy to visualize the abnormal communication between the ureter and the vagina.

✓ Correct Answer: B. Ureterovaginal fistula