**Question:** A 23-year-old man notices new low back pain, stiffness, and left eye discomfort. Sunlight also bothers his eyes. The back pain is worse at night and described as a dull ache in the back and buttock area. On physical examination, there is paravertebral muscle, iliac crest, and ischial tuberosity tenderness with limited flexion of the lumbar spine. His eye is inflamed and the pupil is constricted. Pelvic x-rays show sacroiliitis. Which of the following is the most likely diagnosis for his eye symptoms (it is the most common extra-articular manifestation of this condition)?

A. Rheumatoid arthritis
B. Ankylosing spondylitis
C. Psoriatic arthritis
D. Reactive arthritis

**Correct Answer:** D. Reactive arthritis

**Core Concept:** Reactive arthritis is a type of inflammatory arthritis that can occur following a bacterial or viral infection in the gastrointestinal, genitourinary, or respiratory tracts. It is characterized by the inflammation of the joints, eyes, and other extra-articular sites, including the rectum, urethra, and eyes.

**Why the Correct Answer is Right:**

The correct answer is D. Reactive arthritis because the patient presents with the most common extra-articular manifestation of this condition, which is eye inflammation. Other options are not relevant in this context.

**Why Other Options are Incorrect:**

A. Rheumatoid arthritis is an autoimmune disorder affecting multiple joints symmetrically, with no involvement of the eyes.
B. Ankylosing spondylitis primarily affects the axial skeleton, including the spine, sacroiliac joints, and the pubic symphysis, and typically causes pain in the lower back, buttocks, and sacroiliac joint. It does not affect the eyes.
C. Psoriatic arthritis is characterized by skin lesions, nail pitting, and involvement of the peripheral joints, which is different from the given patient's symptoms.

**Clinical Pearl:**

Reactive arthritis is a rare condition, but knowledge of this entity is crucial for diagnosing patients with eye inflammation and pain in the lower back, buttocks, and sacroiliac joint following a gastrointestinal, genitourinary, or respiratory tract infection. It is essential for clinicians to recognize this as it can lead to misdiagnosis and inappropriate treatment if not diagnosed correctly.