**Core Concept:** Achalasia cardia is a rare disorder characterized by the inability of the lower esophageal sphincter (LES) to relax and the inability of the lower esophageal body to contract, leading to dysphagia and regurgitation of food undigested.

**Why the Correct Answer is Right:** Achalasia cardia is caused by a deficiency in the neurotransmitter substance P, which is responsible for the relaxation of the LES and the peristalsis of the lower esophageal body. This deficiency leads to impaired sensory and motor function of the lower esophagus, causing the characteristic symptoms and signs.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** This answer is incorrect because it does not explain the primary cause of achalasia cardia. Substance P deficiency is the key factor in the pathogenesis of this condition.

B. **Option B:** Although the involvement of the autonomic nervous system is mentioned, this answer is incorrect because it does not emphasize the role of substance P deficiency in the pathophysiology of achalasia cardia.

C. **Option C:** This answer is incorrect as it does not highlight the crucial role of substance P deficiency in the impairment of lower esophageal sphincter relaxation and peristalsis.

D. **Option D:** While this answer mentions the involvement of the autonomic nervous system, it is incorrect because substance P deficiency is the primary cause of achalasia cardia.

**Clinical Pearl:** Achalasia cardia is commonly associated with primary pulmonary hypertension, but this association does not explain the pathogenesis of the condition. Substance P deficiency is the key factor in the development of achalasia cardia, involving the impairment of lower esophageal sphincter relaxation and peristalsis.