What smooth muscle forms the wall of the bladder?

• A. detrusor muscle
• B. internal urethral sphincter
• C. longitudinal muscle
• D. circular muscle

Answer: (A)

Detrusor muscle forms the wall of the bladder. It’s a thick coat of smooth muscle arranged in several layers, which lets the bladder expand as it fills and then contract forcefully to push urine out during urination. The wall isn’t made by just circular or just longitudinal fibers alone—the detrusor includes fibers in multiple directions to coordinate a strong, smooth squeeze. The internal urethral sphincter is a separate smooth muscle at the bladder neck that helps regulate flow, while circular and longitudinal describe the orientations of fibers within the detrusor, not the wall by itself.