This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
A 52 year old lady with a vesicovaginal fistula after abdominal hysterectomy is not responding to conservative management. Most useful and impoant next investigation is: 
Select and explain. 
A. Triple Swab Test, B. Urine culture, C. Cystoscopy, D. IVP
C: Cystoscopy
Answer: C: Cystoscopy 
Explanation: The most valuable investigation in an established case of a vesicovaginal fistula should be cystoscopy. Ref: Female Pelvic Health and Reconstructive Surgery By Carlin and Leong, 1st Edition; Minute Obstetrics and Gynecology consult, 2008, Page 210; Urogynecology and Female Pelvic Reconstructive Surgery : Just the Facts By Sam Siddighi, Jeffrey S. Hardesty, Page 190.