## **Core Concept**
The patient in this scenario is on long-term steroid therapy, which can lead to **avascular necrosis (AVN)**, particularly in weight-bearing joints like the hip. AVN occurs due to the interruption of blood supply to the bone, leading to bone necrosis. Steroid use is a well-known risk factor for AVN.

## **Why the Correct Answer is Right**
The correct answer, **MRI (Magnetic Resonance Imaging)**, is the imaging modality of choice for diagnosing AVN of the hip. MRI is highly sensitive and specific for detecting early changes in bone marrow and can identify the characteristic changes of AVN, such as the "double-line sign" on T2-weighted images. It provides detailed images of the femoral head and surrounding bone marrow, allowing for early diagnosis and assessment of the extent of necrosis.

## **Why Each Wrong Option is Incorrect**
- **Option A: X-ray**. While an X-ray is often the initial imaging done for hip pain, it may not show changes until late in the disease process. Early AVN might not be visible on X-ray.
- **Option B: CT (Computed Tomography) Scan**. CT scans can provide more detail than X-rays but are not as sensitive as MRI for detecting early AVN. CT is better for assessing bone structure but lacks the sensitivity for marrow changes.
- **Option D: Ultrasound**. Ultrasound is not typically used for evaluating AVN of the hip. It is more useful for soft tissue assessment and guiding injections rather than evaluating bone or marrow.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that AVN is a significant concern in patients on long-term steroid therapy. Early diagnosis with MRI can help in management and potentially prevent collapse of the femoral head. Steroid-induced AVN is often bilateral, so imaging both hips is crucial.

## **Correct Answer:** . MRI.