**Core Concept**
Acute upper GI bleeding can result from various causes, including mucosal lesions, vascular abnormalities, and other conditions. Understanding the relative frequencies of these causes is essential for effective diagnosis and management.

**Why the Correct Answer is Right**
Vascular ectasia, also known as angiodysplasia, is a rare cause of upper GI bleeding. It is characterized by the presence of abnormal, fragile blood vessels in the GI tract, which are prone to rupture and bleeding. Unlike other causes of upper GI bleeding, such as ulcers or varices, vascular ectasia is less common and often presents in older adults. The exact pathophysiology of vascular ectasia is not fully understood, but it is thought to result from chronic inflammation and angiogenesis in the affected vessels.

**Why Each Wrong Option is Incorrect**
**Option A:** Mallory Weiss tear is a mucosal tear that occurs at the junction of the stomach and esophagus, often due to intense vomiting or retching. While it is a potential cause of upper GI bleeding, it is not the least common cause.
**Option B:** Peptic ulcers are a common cause of upper GI bleeding, resulting from the erosion of the mucosal lining of the stomach or duodenum. They are often associated with Helicobacter pylori infection or the use of nonsteroidal anti-inflammatory drugs (NSAIDs).
**Option C:** Esophageal varices are dilated submucosal veins in the esophagus, often resulting from liver cirrhosis and portal hypertension. They are a common cause of upper GI bleeding in patients with liver disease.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with upper GI bleeding, it is essential to consider the patient's age, medical history, and underlying conditions. Vascular ectasia is more common in older adults and may be associated with a history of chronic kidney disease or liver disease.

**✓ Correct Answer: D. Vascular ectasia**