**Core Concept**
The question is testing knowledge of obstetric injuries that can lead to uretero vaginal fistula, a type of genitourinary fistula. A uretero vaginal fistula occurs when there is an abnormal connection between the ureter and the vagina, often as a result of surgical or traumatic injury.

**Why the Correct Answer is Right**
The most common site of obstetric injury leading to uretero vaginal fistula is the lower third of the vagina. This is because the ureters enter the bladder at an angle, making them more susceptible to injury, particularly during vaginal delivery or cesarean section. The lower third of the vagina is a common site for obstetric injury due to the proximity of the ureters to the vaginal wall.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the upper third of the vagina is not a common site for obstetric injury leading to uretero vaginal fistula. The upper third of the vagina is less likely to be injured during vaginal delivery or cesarean section.
**Option B:** This option is incorrect because the bladder is not a common site for obstetric injury leading to uretero vaginal fistula. While the bladder can be injured during obstetric procedures, the ureters are more likely to be injured, leading to a uretero vaginal fistula.
**Option C:** This option is incorrect because the urethra is not a common site for obstetric injury leading to uretero vaginal fistula. The urethra can be injured during childbirth, but this is less likely to lead to a uretero vaginal fistula.

**Clinical Pearl / High-Yield Fact**
A uretero vaginal fistula can present with symptoms such as incontinence, urinary frequency, and hematuria. It is essential to diagnose and treat this condition promptly to prevent long-term complications.

**Correct Answer: D. Lower third of the vagina.**