## **Core Concept**
Ankylosing spondylitis (AS) is a type of chronic inflammatory arthritis primarily affecting the spine and sacroiliac joints, leading to significant stiffness and potential fusion of vertebrae. Early diagnosis is crucial for effective management and prevention of long-term complications. The diagnostic approach involves a combination of clinical assessment, laboratory tests, and imaging studies.

## **Why the Correct Answer is Right**
The correct answer, **X-ray of sacroiliac joints**, is the earliest investigation for diagnosing ankylosing spondylitis because it directly visualizes the sacroiliac joints, which are typically involved early in the disease process. Radiographic findings may include erosions, sclerosis, and joint space narrowing. MRI is highly sensitive for detecting early inflammatory changes but is usually reserved for cases with high suspicion of AS and normal X-rays or to assess disease activity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While MRI can detect early inflammatory changes in the sacroiliac joints and spine, it is not typically used as the first-line imaging modality due to cost, availability, and the fact that X-ray findings can often provide sufficient early diagnostic clues.
- **Option B:** CT scan of the spine provides detailed images of bone structures but is not the first choice for diagnosing AS due to radiation exposure and the fact that it does not offer significant advantages over X-ray in early diagnosis.
- **Option D:** Ultrasound of the spine is not commonly used for diagnosing AS as it does not provide a clear view of the sacroiliac joints or the spine's bony structures.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the Assessment of Spondyloarthritis international Society (ASAS) classification criteria emphasize the importance of sacroiliitis on imaging (either X-ray or MRI) as a critical component for diagnosing non-radiographic axial SpA and AS. The presence of chronic back pain and stiffness, especially in young males, should prompt an early investigation with X-rays of the sacroiliac joints.

## **Correct Answer:** . X-ray of sacroiliac joints