## Core Concept
The question pertains to the complications of peptic ulcers, which include bleeding, perforation, obstruction, and penetration. These complications arise due to the erosive nature of gastric acid on the mucosal lining of the stomach or duodenum.

## Why the Correct Answer is Right
Gastric perforation is a critical and life-threatening complication where the ulcer erodes through the stomach or duodenal wall, leading to peritonitis. Bleeding is another common complication due to erosion into blood vessels. Obstruction can occur due to swelling and scarring that narrows the lumen. Penetration refers to the ulcer eroding into adjacent organs. Among these, while all are significant, **penetration** is considered less common compared to the others like bleeding, perforation, and obstruction.

## Why Each Wrong Option is Incorrect
- **Option A:** Bleeding is one of the most common complications of peptic ulcers, resulting from the ulcer's erosion into a blood vessel.
- **Option B:** Perforation is also a well-known and serious complication where the ulcer burns through the wall of the stomach or intestine.
- **Option C:** Obstruction, though less common than bleeding or perforation, occurs due to swelling and scarring from the ulcer, which can narrow the passageway.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while all complications of peptic ulcers are serious, **penetration** into adjacent organs (like the pancreas or liver) is less frequently encountered compared to the other listed complications. This differentiation is crucial for diagnosis and management.

## Correct Answer: D. Penetration