**Core Concept**
A false positive fistula test refers to a situation where the test result indicates the presence of a fistula when, in fact, one is not present. This is often due to an abnormal communication between the urinary and genital systems, or between the urinary system and the peritoneal cavity.

**Why the Correct Answer is Right**
The most common cause of a false positive fistula test is a ureterocele, which is a cystic dilatation of the terminal ureter. This can occur when the ureteral orifice is obstructed, leading to the accumulation of urine and the formation of a cystic structure. In some cases, the ureterocele can be associated with a ectopic ureter, which is a congenital anomaly where the ureter opens into the bladder or urethra in an abnormal location.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not describe a common cause of false positive fistula test. While a vesicoureteral reflux can lead to urinary tract infections and other complications, it is not typically associated with a false positive fistula test.
* **Option B:** This option is incorrect because it does not describe a condition that would lead to a false positive fistula test. A urethral stricture is a narrowing of the urethra that can cause obstructive symptoms, but it is not typically associated with a false positive fistula test.
* **Option C:** This option is incorrect because it does not describe a condition that would lead to a false positive fistula test. A bladder tumor can cause a variety of symptoms, including hematuria and urinary frequency, but it is not typically associated with a false positive fistula test.

**Clinical Pearl / High-Yield Fact**
A ureterocele can be associated with a variety of symptoms, including urinary tract infections, hematuria, and flank pain. In some cases, a ureterocele can be diagnosed prenatally, and surgical intervention may be necessary to prevent complications.

**Correct Answer: C. Ureterocele.**