**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 in swallowing and regurgitation of food. The pathophysiology involves an abnormality in the neural control of the LES, resulting in a non-relaxing LES and impaired peristalsis in the esophagus.

**Why the Correct Answer is Right**
The treatment of choice for achalasia cardia is a drug that relaxes the LES, allowing for easier passage of food into the stomach. The drug that achieves this is **nitroglycerin**, which acts as a smooth muscle relaxant. Nitroglycerin is a vasodilator that releases nitric oxide, which activates guanylate cyclase, leading to a decrease in intracellular calcium levels and relaxation of smooth muscle cells. This results in a decrease in the resting pressure of the LES, allowing for easier passage of food.

**Why Each Wrong Option is Incorrect**
* **Option A:** Metoclopramide is a prokinetic agent that enhances gastric emptying and motility, but it does not directly relax the LES.
* **Option B:** Omeprazole is a proton pump inhibitor that reduces gastric acid secretion, but it does not treat the underlying motility disorder of achalasia cardia.
* **Option C:** Baclofen is a GABA receptor agonist that can decrease LES pressure, but it is not the first-line treatment for achalasia cardia.

**Clinical Pearl / High-Yield Fact**
The "Triple Therapy" for achalasia cardia involves combining nitrates (e.g., nitroglycerin), calcium channel blockers (e.g., nifedipine), and endoscopic dilation to effectively relax the LES and improve esophageal emptying.

**Correct Answer: D. Nitroglycerin**