**Core Concept**
The cardioesophageal sphincter, also known as the lower esophageal sphincter (LES), is a critical component of the esophageal-gastric junction. It plays a crucial role in preventing the reflux of gastric contents into the esophagus by maintaining a high pressure gradient between the esophagus and the stomach.

**Why the Correct Answer is Right**
The correct answer will be related to a characteristic of the cardioesophageal sphincter that is not true. The LES is a functional sphincter, not a muscular one, and its primary function is to maintain a high pressure gradient to prevent reflux. The LES is innervated by both the enteric nervous system and the autonomic nervous system, which helps regulate its tone and function. The LES also receives inhibitory input from the vagus nerve, which helps to relax the sphincter and facilitate swallowing.

**Why Each Wrong Option is Incorrect**
**Option A:** The LES is a muscular sphincter - This statement is incorrect because the LES is actually a functional sphincter, not a muscular one. It is formed by the thickened mucosa and the longitudinal muscle layer of the distal esophagus, which contracts to increase pressure and prevent reflux.

**Option B:** The LES is only innervated by the enteric nervous system - This statement is incorrect because the LES is actually innervated by both the enteric nervous system and the autonomic nervous system. The enteric nervous system provides intrinsic innervation, while the autonomic nervous system provides extrinsic innervation that helps regulate the tone and function of the LES.

**Option C:** The LES is only relaxed by the vagus nerve - This statement is incorrect because the LES is actually relaxed by both the vagus nerve and the enteric nervous system. The vagus nerve provides inhibitory input that helps to relax the LES and facilitate swallowing.

**Option D:** The LES is a passive sphincter - This statement is incorrect because the LES is actually an active sphincter that plays a crucial role in preventing reflux. It is formed by the thickened mucosa and the longitudinal muscle layer of the distal esophagus, which contracts to increase pressure and prevent reflux.

**Clinical Pearl / High-Yield Fact**
The LES is a critical component of the esophageal-gastric junction, and its dysfunction can lead to conditions such as gastroesophageal reflux disease (GERD). The LES is also sensitive to changes in intragastric pressure, and its function can be affected by factors such as obesity, pregnancy, and certain medications.

**Correct Answer: D. The LES is a passive sphincter.**