This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thorax, Subject name: Anatomy
Which muscle causes opening of the upper end of esophagus? 
Select and explain. 
A. Epiglottis, B. Thyropharungeus, C. Stylopharyngeus, D. Cricopharyngeus of inferior constrictor
D: Cricopharyngeus of inferior constrictor
Answer: D: Cricopharyngeus of inferior constrictor 
Explanation: Ans. is 'd' i.e., Cricopharyngeus of inferior constrictor Upper esophageal sphinctor* The upper esophageal sphincter surrounds the upper part of the esophagus.* It consists of skeletal muscle, but is not under voluntary control.* Opening of the upper esophageal sphincter is triggered by the swallowing reflex.* The primary muscle of the upper esophageal sphincter is the cricopharyngeal part of the inferior pharyngeal constrictor.