**Core Concept:** Peptic ulcer is a break in the protective mucosal lining of the stomach or duodenum (first part of the small intestine) resulting from an imbalance between aggressive factors (e.g., acid, pepsin) and protective factors (e.g., mucus, bicarbonate secretion).

**Why the Correct Answer is Right:** The correct answer is "Option D" because peptic ulcers are primarily caused by the imbalance between aggressive and protective factors, particularly in the context of Helicobacter pylori infection, nonsteroidal anti-inflammatory drug (NSAID) use, and smoking.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** Peptic ulcers can occur due to genetic factors, stress, or the use of corticosteroids. Although these factors may contribute to ulcer development, they are not specifically associated with the primary causes mentioned above (H. pylori, NSAIDs, and smoking).

B. **Option B:** Infection with other organisms like Helicobacter pylori is a common cause of peptic ulcers. This option is incorrect because it focuses on a specific cause, rather than the broader concept of the imbalance between aggressive and protective factors.

C. **Option C:** NSAIDs are a risk factor for peptic ulcer, but they are not the primary cause. This option is incorrect for the same reason as Option B.

**Clinical Pearl:** A high index of suspicion and appropriate workup are crucial when evaluating patients with suspected peptic ulcer disease, as early diagnosis and management can prevent complications like bleeding and perforation.

**Correct Answer:** D.