**Core Concept**
Reiter's syndrome, also known as reactive arthritis, is a type of arthritis that occurs as a reaction to an infection elsewhere in the body, typically in the gastrointestinal tract or the urethra. The classic triad of Reiter's syndrome includes **arthritis**, **urethritis**, and **conjunctivitis**.

**Why the Correct Answer is Right**
The correct answer is the option that does not belong to the classic triad of Reiter's syndrome. Since the question is incomplete, let's assume the options are: A. Arthritis, B. Urethritis, C. Conjunctivitis, and D. Oral ulcers. The triad consists of arthritis, urethritis, and conjunctivitis, making oral ulcers the correct answer.

**Why Each Wrong Option is Incorrect**
**Option A:** Arthritis is a key component of Reiter's syndrome, characterized by joint inflammation and pain.
**Option B:** Urethritis, or inflammation of the urethra, is another part of the classic triad.
**Option C:** Conjunctivitis, an eye infection, is the third element of the triad.

**Clinical Pearl / High-Yield Fact**
Reiter's syndrome is often associated with **HLA-B27**, a genetic marker that increases the risk of developing this condition.

**Correct Answer:** D. Oral ulcers