This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal System, Subject name: Physiology
Lower esophageal sphincter 
Select and explain. 
A. Has no tonic activity, B. Has a tone which is provided by the sympathetic system, C. Relaxes on increasing abdominal pressure, D. Relaxes ahead of the peristaltic wave
D: Relaxes ahead of the peristaltic wave
Answer: D: Relaxes ahead of the peristaltic wave 
Explanation: Lower esophageal sphincter (LES) The resting pressure is about 20 mm Hg. Basal tone regulated by both intrinsic and extrinsic nerves, and hormones. A significant fraction of this basal tone is mediated by vagal cholinergic fibers (contraction of LES). Hence, options A & B are not correct. Stimulation of sympathetic nerves to the sphincter causes the LES to contract. Relaxation LES occurs in response to primary peristalsis in the esophagus is mediated by VIP and NO. Contraction of the diaphragm or increased intra-abdominal pressure will increase the pressure in LES. Option C is not correct.