**Core Concept**
The patient's presentation of severe pain in the right hip following long-term steroid therapy is suggestive of avascular necrosis (AVN), a condition characterized by the death of bone tissue due to a lack of blood supply. This condition is a known complication of corticosteroid therapy, particularly with prolonged use.

**Why the Correct Answer is Right**
The imaging modality of choice for diagnosing AVN is MRI, as it provides high-resolution images of the bone marrow and surrounding soft tissues. MRI is particularly useful in detecting early changes in the bone marrow, such as edema and necrosis, which are characteristic of AVN. Additionally, MRI is non-invasive and does not expose the patient to radiation. The sensitivity and specificity of MRI for diagnosing AVN are also high, making it the preferred imaging modality for this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Although X-ray can show late changes of AVN, such as collapse of the femoral head, it is not the imaging modality of choice for early detection.
**Option B:** CT scan can provide detailed images of the bone, but it is not as sensitive as MRI for detecting early changes in the bone marrow.
**Option C:** Ultrasound is not typically used for diagnosing AVN, as it is not as effective in visualizing the bone marrow and surrounding soft tissues.

**Clinical Pearl / High-Yield Fact**
It is essential to suspect AVN in patients on long-term steroid therapy who present with hip pain, as early diagnosis and treatment can significantly improve outcomes.

**Correct Answer: A. MRI**