**Core Concept**
Achalasia cardia is a primary esophageal motility disorder characterized by the failure of the lower esophageal sphincter (LES) to relax, leading to difficulty swallowing and regurgitation of food. This condition results from degeneration of the myenteric plexus, which is responsible for the coordination of esophageal peristalsis.

**Why the Correct Answer is Right**
The degeneration of the myenteric plexus in achalasia cardia leads to an uncoordinated and non-peristaltic contraction of the esophageal smooth muscle. This results in a sustained contraction of the LES, making it difficult for food to pass into the stomach. The loss of inhibitory neurons in the myenteric plexus, particularly those containing nitric oxide, further contributes to the failure of LES relaxation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because achalasia cardia is not primarily caused by a mechanical obstruction, but rather a functional disorder of the LES.
**Option B:** This option is incorrect because the treatment of achalasia cardia involves interventions aimed at relaxing the LES, such as pneumatic dilation or peroral endoscopic myotomy (POEM), rather than increasing its tone.
**Option C:** This option is incorrect because the primary symptom of achalasia cardia is dysphagia, which is the difficulty swallowing food, rather than regurgitation of food.

**Clinical Pearl / High-Yield Fact**
Achalasia cardia should be distinguished from other causes of dysphagia, such as esophageal stricture or carcinoma, which may present with similar symptoms but have different underlying pathologies.

**Correct Answer:** C.