**Core Concept**
The upper end of the esophagus is controlled by a specific muscle that regulates the passage of food from the pharynx to the esophagus. This muscle is part of the pharyngeal constrictor group, which is responsible for the swallowing process.

**Why the Correct Answer is Right**
The cricopharyngeus muscle, located at the upper end of the esophagus, is a component of the inferior constrictor muscle of the pharynx. This muscle plays a crucial role in the initiation of swallowing by relaxing and allowing the upper end of the esophagus to open, thus facilitating the passage of food from the pharynx into the esophagus. The cricopharyngeus muscle is innervated by the pharyngeal plexus, which is a complex network of nerves that regulates various functions of the pharynx.

**Why Each Wrong Option is Incorrect**
**Option A:** The epiglottis is a flap-like structure that separates the trachea from the esophagus during swallowing, preventing food from entering the airway. It does not control the opening of the upper end of the esophagus.

**Option B:** The thyropharyngeus muscle is a component of the inferior constrictor muscle of the pharynx, but it primarily assists in the constriction of the pharynx during swallowing, rather than regulating the opening of the upper end of the esophagus.

**Option C:** The stylopharyngeus muscle is a muscle of the pharynx that elevates the pharynx and larynx during swallowing. However, it does not control the opening of the upper end of the esophagus.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the cricopharyngeus muscle is a key regulator of the upper end of the esophagus, and its dysfunction can lead to conditions such as achalasia or pharyngeal diverticula.

**✓ Correct Answer: D. Cricopharyngeus of inferior constrictor. The cricopharyngeus muscle is responsible for the opening of the upper end of the esophagus.**