**Core Concept**
Downhill esophageal varices are a type of esophageal varix that occurs due to the increased pressure gradient between the hepatic veins and the inferior vena cava, leading to the formation of collateral veins in the esophagus. This is a result of portal hypertension, often seen in liver cirrhosis.

**Why the Correct Answer is Right**
The correct answer is because downhill esophageal varices appear as a filling defect or a "cobra head" sign on a barium swallow study, which is indicative of a varix. This occurs when the barium column flows through the esophagus and creates a filling defect due to the varix. The "cobra head" sign is named due to its resemblance to the head of a cobra, with the varix appearing as the head and the body of the snake being the esophagus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a clear description of how downhill esophageal varices appear on a barium swallow study.
**Option B:** This option is incorrect because it may be describing a different type of esophageal abnormality, such as a stricture or a tumor.
**Option C:** This option is incorrect because it is not a commonly used description for the appearance of downhill esophageal varices on a barium swallow study.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that downhill esophageal varices are a type of complication that can occur in patients with portal hypertension, and their appearance on imaging studies, such as barium swallow, can be an important diagnostic clue.

**Correct Answer: B. "Cobra head" sign.**