**Core Concept**
The patient's presentation of chronic low back pain, stiffness, and sacroiliitis is suggestive of ankylosing spondylitis (AS), a type of spondyloarthritis characterized by inflammation of the sacroiliac joints and axial skeleton. This condition involves a complex interplay between genetic predisposition, environmental factors, and immune system dysregulation.

**Why the Correct Answer is Right**
The next step in management involves assessing the patient's risk of developing new bone formation and fusion, which can lead to significant morbidity. MRI of the spine and sacroiliac joints is essential to evaluate the extent of inflammation and potential new bone formation. This imaging modality provides valuable information on the presence of synovitis, enthesitis, and bone marrow edema, which are characteristic of AS. The MRI findings will guide the management plan, including the initiation of disease-modifying antirheumatic drugs (DMARDs) and close monitoring of the disease activity.

**Why Each Wrong Option is Incorrect**
**Option A:** A plain radiograph of the lumbar spine is not sufficient to diagnose AS, as it may not show early changes. In fact, radiographs may appear normal in early stages of the disease or show only mild changes, such as sacroiliitis.

**Option B:** While physiotherapy and exercise are essential in the management of AS, they do not replace the need for imaging to assess the extent of disease activity and potential new bone formation.

**Option C:** Starting DMARDs without imaging to assess disease activity and potential new bone formation may not be the most appropriate approach, as it may lead to unnecessary treatment and potential side effects.

**Option D:** Ordering a CT scan of the lumbar spine is not the most appropriate next step, as it may expose the patient to unnecessary radiation and does not provide the same level of detail as MRI in evaluating disease activity and potential new bone formation.

**Clinical Pearl / High-Yield Fact**
In patients with AS, it is essential to regularly monitor disease activity and assess the risk of developing new bone formation and fusion, as this can significantly impact the patient's quality of life and mobility.

**Correct Answer:** C.