What reflex occurs when the detrusor muscle contracts and causes the internal sphincter to relax?

The micturition reflex is the correct answer because it describes the physiological process that occurs during urination. When the bladder fills with urine, stretch receptors in the bladder wall are activated, sending signals to the spinal cord and brain. This reflex causes the detrusor muscle, which is the smooth muscle of the bladder, to contract. Simultaneously, it signals the internal sphincter to relax, allowing urine to flow from the bladder through the urethra and out of the body. This coordinated action is essential for the process of urination, enabling the voluntary and involuntary aspects of this physiological response.

Other responses relate to different processes—urinary retention refers to the inability to empty the bladder, voiding reflex can refer to the broader neural pathways involved in initiating urination but doesn’t capture the entire event, and the defecation reflex pertains to bowel movements, which is an entirely separate process involving the intestines and rectum. Thus, the specific steps described in the micturition reflex distinctively identify the contraction of the detrusor muscle and relaxation of the internal sphincter as crucial to successfully allowing urine to exit the bladder.