**Core Concept**
The patient's symptoms of nonprogressive dysphagia for both solids and liquids suggest an esophageal motility disorder. The barium swallow test is a diagnostic imaging modality that can help identify abnormalities in the esophagus, such as strictures, tumors, or motility disorders.

**Why the Correct Answer is Right**
The characteristic finding on barium swallow that will confirm the probable diagnosis of achalasia is a **bird's beak appearance**. This is due to the failure of the lower esophageal sphincter (LES) to relax, causing a narrowing of the distal esophagus. The barium swallow test will show a smooth, tapering narrowing of the distal esophagus, resembling a bird's beak. The pathophysiology involves the degeneration of the myenteric plexus, leading to a failure of peristalsis and a non-relaxing LES.

**Why Each Wrong Option is Incorrect**
**Option A:** A stricture would show a narrow, irregular segment of the esophagus on barium swallow, but it would not have the characteristic bird's beak appearance.

**Option B:** A Zenker's diverticulum would show a diverticulum in the upper esophagus, but it would not cause nonprogressive dysphagia for both solids and liquids.

**Option C:** A hiatal hernia would show a herniation of the stomach into the thorax on barium swallow, but it would not cause nonprogressive dysphagia for both solids and liquids.

**Clinical Pearl / High-Yield Fact**
Achalasia is a rare esophageal motility disorder that can be treated with pneumatic dilation or Heller myotomy. Patients with achalasia often have a characteristic voice, known as a "hot potato voice," due to the difficulty in swallowing.

**Correct Answer:** . Bird's beak appearance.