**Core Concept**
Sacro-iliac joint involvement is a hallmark feature of spondyloarthritis, particularly in Ankylosing Spondylitis (AS), which is a chronic inflammatory disease affecting the axial skeleton, including the sacroiliac joints.

**Why the Correct Answer is Right**
Ankylosing spondylitis is characterized by chronic inflammation of the sacroiliac joints, often presenting with early back pain and stiffness, especially in the morning. The pathophysiology involves HLA-B27 positivity, immune-mediated inflammation, and enthesitis (inflammation at tendon/ligament insertion sites). Radiographic changes such as sacroiliitis are diagnostic and occur early in the disease. This joint involvement is highly specific and distinguishes AS from other arthritides.

**Why Each Wrong Option is Incorrect**
Option B: Rheumatoid arthritis primarily affects peripheral joints (e.g., hands, knees) and is typically symmetrical, with synovial inflammation and rheumatoid factor positivity. Sacroiliac joint involvement is rare and not a feature of RA.
Option C: Reiter's syndrome (now part of reactive arthritis) involves uveitis, conjunctivitis, and arthritis, but sacroiliac joint involvement is not typical or prominent.
Option D: Osteoarthritis is a degenerative joint disease involving cartilage loss and joint space narrowing, with no inflammatory component. It rarely affects sacroiliac joints and is not associated with inflammatory markers.

**Clinical Pearl / High-Yield Fact**
Sacroiliac joint pain and stiffness in the morning that improves with activity is a classic red flag for Ankylosing Spondylitis — especially in young males with HLA-B27 positivity. Early recognition is critical for initiating anti-inflammatory therapy and preventing spinal fusion.

✓ Correct Answer: A. Ankylosing spondylitis