**Core Concept**
Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the axial skeleton, characterized by sacroiliitis, spinal stiffness, and eventual fusion of vertebrae. Early diagnosis of AS is crucial for timely management and prevention of complications.

**Why the Correct Answer is Right**
The earliest investigation for diagnosing AS is the **plain radiograph of the sacroiliac joint (SIJ)**. This imaging modality is sensitive to changes in the SIJ, such as erosions, sclerosis, and fusion, which are characteristic of AS. The presence of these changes, especially in the presence of clinical symptoms, can help establish an early diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** MRI of the spine is useful for detecting inflammatory changes, but it is not the primary investigation for diagnosing AS. MRI can show changes in the spine, but these are not as specific as those seen in the SIJ.
**Option B:** CT scans of the spine can provide detailed images of the vertebrae and sacrum but are not the first-line investigation for AS.
**Option C:** Blood tests, such as HLA-B27 testing, can be helpful in confirming the diagnosis of AS but are not the primary investigation.

**Clinical Pearl / High-Yield Fact**
In AS, the **HLA-B27 antigen** is present in approximately 90% of patients, but a positive test result does not confirm the diagnosis, as the antigen can also be present in healthy individuals.

**Correct Answer:** A. Plain radiograph of the sacroiliac joint.