**Core Concept**
The question requires the selection of an appropriate imaging modality for diagnosing avascular necrosis (AVN) of the femoral head, a common complication of long-term steroid therapy. AVN is characterized by the death of bone tissue due to interrupted blood supply.

**Why the Correct Answer is Right**
The imaging modality of choice for AVN is MRI (Magnetic Resonance Imaging), which provides detailed images of the bone marrow and surrounding soft tissues. MRI is highly sensitive for detecting early changes of AVN, such as marrow edema and fatty infiltration. It can also help differentiate AVN from other hip pathologies, such as osteoarthritis or fractures.

**Why Each Wrong Option is Incorrect**
* **Option A:** X-ray is not the best initial choice for diagnosing AVN, as it may not show changes until late in the disease process. X-rays are better suited for detecting osteoarthritic changes or hip fractures.
* **Option B:** CT (Computed Tomography) scans can detect bony changes, but they are not as sensitive as MRI for early AVN. CT scans also involve higher radiation doses.
* **Option D:** Ultrasound is not typically used for diagnosing AVN, as it is not as effective for imaging bone marrow and is more suited for soft tissue injuries.

**Clinical Pearl / High-Yield Fact**
For patients on long-term steroid therapy, regular follow-up with imaging studies (preferably MRI) is crucial for early detection and management of AVN.

**Correct Answer:** C. MRI.