**Core Concept**
Achalasia cardia is a primary motility disorder of the esophagus characterized by the failure of normal peristalsis and incomplete relaxation of the lower esophageal sphincter (LES) during swallowing. This results in impaired esophageal emptying and symptoms such as dysphagia, regurgitation, and chest pain.

**Why the Correct Answer is Right**
In achalasia cardia, the LES fails to relax properly, leading to an elevated resting pressure, typically greater than 30 mmHg. Additionally, the absence of normal peristalsis in the esophageal body contributes to the impaired esophageal emptying. The increased pressure at the distal end of the esophagus is a direct result of the LES's failure to relax, leading to a functional obstruction. The degeneration of the myenteric plexus, a process known as denervation, also contributes to the development of achalasia.

**Why Each Wrong Option is Incorrect**
**Option B:** Low pressure at LES with no peristalsis - This is incorrect because achalasia cardia is characterized by high pressure at the LES, not low pressure. Additionally, while there is a lack of peristalsis, the LES pressure is elevated.
**Option C:** Pressure > 50 mmHg with peristalsis - This is incorrect because achalasia cardia is characterized by a lack of peristalsis, not the presence of it.
**Option D:** Pressure at the distal end increased with normal relaxation - This is incorrect because in achalasia cardia, the LES does not relax normally during swallowing.

**Clinical Pearl / High-Yield Fact**
Achalasia cardia is often associated with the absence of peristalsis and an elevated resting pressure at the LES, which can be further evaluated using high-resolution manometry. The diagnosis can be challenging, and a combination of clinical evaluation, endoscopy, and manometry is often necessary for a definitive diagnosis.

**✓ Correct Answer: A. Pressure at distal end increased with no peristalsis**