**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, often in the gastrointestinal tract or the urogenital tract. It is characterized by the classic triad of arthritis, urethritis, and conjunctivitis.

**Why the Correct Answer is Right**
The correct answer is one of the options that does not fit within the classic triad of Reiter's syndrome. This syndrome is typically seen in individuals who have recently been infected with Chlamydia trachomatis or other sexually transmitted pathogens. The condition is mediated by an autoimmune response to the infection, which leads to inflammation in the joints, eyes, and urethra.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because urethritis is a hallmark symptom of Reiter's syndrome, often presenting as dysuria or urethral discharge.
* **Option B:** This option is incorrect because conjunctivitis is a common feature of Reiter's syndrome, presenting as redness, swelling, and discharge in the eyes.
* **Option C:** This option is incorrect because arthritis is the primary symptom of Reiter's syndrome, often presenting as joint pain, swelling, and stiffness in the knees, ankles, and other joints.

**Clinical Pearl / High-Yield Fact**
Reiter's syndrome is a classic example of an autoimmune response to an infection, and its symptoms can be remembered using the mnemonic "ARCO": Arthritis, Urethritis, Conjunctivitis, and Oral lesions (such as ulcers or mouth sores).

**Correct Answer: D.**