**Core Concept**
The question is testing the knowledge of the common causes of proximal lytic lesions of the head of the femur, particularly in a 13-year-old boy. A lytic lesion is an area of bone destruction, and identifying the cause is crucial for appropriate management. In this context, the head of the femur is a common location for various bone disorders.

**Why the Correct Answer is Right**
The correct answer is a rare cause of proximal lytic lesions. Most common causes of lytic lesions in this location include:
* **Fibrous cortical defect** (also known as a non-ossifying fibroma): a benign bone tumor that often presents as a lytic lesion in the metaphyseal region of long bones, including the femur.
* **Osteosarcoma**: a primary malignant bone tumor that can cause a lytic lesion in the metaphysis of long bones, including the femur.
* **Ewing's sarcoma**: a highly malignant bone tumor that can cause a lytic lesion in the diaphysis or metaphysis of long bones, including the femur.
* **Giant cell tumor**: a rare, benign bone tumor that often presents as a lytic lesion in the epiphysis of long bones, including the distal femur.

However, a rare cause of proximal lytic lesions of the head of the femur in a 13-year-old boy would be a **Fibrodysplasia ossificans progressiva (FOP)**, which is a rare, genetic disorder characterized by the formation of bone in muscles, tendons, and other soft tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** Fibrous cortical defect is a common cause of lytic lesions in the head of the femur in children, making it an incorrect answer.
**Option B:** Osteosarcoma is a common malignant bone tumor that can cause lytic lesions in the head of the femur, making it an incorrect answer.
**Option C:** Ewing's sarcoma is another common malignant bone tumor that can cause lytic lesions in the head of the femur, making it an incorrect answer.
**Option D:** Giant cell tumor is a rare, benign bone tumor that often presents as a lytic lesion in the epiphysis of long bones, but it is not the least common cause of proximal lytic lesions of the head of the femur.

**Clinical Pearl / High-Yield Fact**
Fibrodysplasia ossificans progressiva (FOP) is a rare, genetic disorder that can cause heterotopic ossification, leading to the formation of bone in muscles, tendons, and other soft tissues. This condition can mimic other bone disorders, making it essential to consider it in the differential diagnosis of proximal lytic lesions.

**Correct Answer: D. Giant cell tumor.**