## **Core Concept**
The patient's long-term use of steroids due to nephrotic syndrome likely led to the development of **steroid-induced osteonecrosis**, also known as avascular necrosis (AVN). This condition commonly affects the femoral head, leading to hip pain and limited mobility.

## **Why the Correct Answer is Right**
The correct answer, **Avascular necrosis of the femoral head**, is the most probable diagnosis given the patient's history of long-term steroid use and symptoms of limp gait and limitation of abduction and internal rotation. Steroid-induced osteonecrosis typically affects the femoral head due to its rich blood supply that is susceptible to steroid-induced damage. The limited mobility, particularly in abduction and internal rotation, is a classic presentation due to the involvement of the femoral head.

## **Why Each Wrong Option is Incorrect**
- **Option B: Fracture of the femoral neck** - While a fracture could cause similar symptoms, the gradual onset and specific limitations in movement, along with the history of steroid use, make avascular necrosis more likely.
- **Option C: Rheumatoid arthritis involving the hip** - Although rheumatoid arthritis can cause hip involvement, the specific history of steroid use and the nature of the symptoms point more towards avascular necrosis.
- **Option D: Degenerative osteoarthritis of the hip** - Degenerative osteoarthritis could cause limitation of movement and pain but would not typically present with such a distinct history of steroid use and would more likely involve other joints as well.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **avascular necrosis of the femoral head** is a significant complication of long-term steroid use. Early diagnosis and management are crucial to prevent further collapse of the femoral head and the need for surgical intervention. Patients on long-term steroids should be monitored for hip pain and limited mobility.

## **Correct Answer:** . Avascular necrosis of the femoral head