Bladder outlet obstruction (BOO) is a blockage at the base of the bladder that reduces or prevents the flow of urine into the urethra, the tube that carries urine out of the body. Bladder outlet obstruction (BOO) can have many different causes, including: Benign prostatic hyperplasia (BPH), or enlarged prostate; Bladder stones; Bladder tumors (cancer); Pelvic tumors (cervix, prostate, uterus, rectum); Urethral stricture (scar tissue); Less common causes include: Cystocele (when the bladder falls into the vagina); Foreign objects; Posterior urethral valves (birth defect in males); Urethral spasms; Urethral diverticula; This condition is most common in aging men. It is often caused by BPH. Bladder stones and bladder cancer are also more commonly seen in men than women. As a man ages, his chance of developing these diseases increases dramatically.