**Core Concept**
Cardiac achalasia is a motility disorder characterized by the failure of the lower esophageal sphincter (LES) to relax, leading to difficulty swallowing and regurgitation of food. The condition is often diagnosed using radiographic studies, such as barium swallow or esophagram, which provide valuable information about the esophageal anatomy and function.

**Why the Correct Answer is Right**
In cardiac achalasia, the LES fails to relax, resulting in a characteristic "bird's beak" appearance on radiographs due to the narrow distal esophagus. This is caused by the sustained contraction of the LES and the failure of the esophageal body to relax. The esophagus is also often dilated proximal to the LES, which can be seen as a "dilated esophageal sac". The radiographic findings are a result of the impaired relaxation of the LES and the resulting esophageal obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a characteristic radiographic finding of cardiac achalasia. In fact, a "corkscrew" appearance is more commonly associated with sigmoid colon or small intestine obstruction.
**Option B:** This option is incorrect because it is not a typical radiographic feature of cardiac achalasia. A "beak-like" appearance is more characteristic of the "bird's beak" sign seen in this condition.
**Option C:** This option is incorrect because it is not a correct radiographic finding of cardiac achalasia. In fact, a "sigmoid colon" appearance is more commonly associated with sigmoid colon or small intestine obstruction.

**Clinical Pearl / High-Yield Fact**
A key feature of cardiac achalasia is the failure of the LES to relax, which can be remembered using the mnemonic "LES" - "Loss of Esophageal Sphincter" relaxation.

**Correct Answer:** D