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: Physiology
Which of the following statement regarding lower esophageal sphincter is TRUE? 
Select and explain. 
A. It has no tonic activity, B. It 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: During swallowing when the peristaltic wave sweep down the esophagus the gastroesophageal sphincter relaxes so that the bolus of food can pass into the stomach. the lower esophageal sphincter is tonically active but relaxes on swallowing. This tonic activity of the LES between meals prevents reflux of gastric contents into the esophagus. its tone is under neural control by the parasympathetic nervous system. Contraction of LES is caused by the release of acetylcholine from the vagus and release of NO and VIP from interneurons innervated by other vagal fibers causes it to relax. Ref: Fundamentals of Human Physiology By Lauralee Sherwood page 447. Ganong's Review of Medical Physiology, 24e CHAPTER 27.