**Core Concept**
Chondromyxoid fibroma is a rare, benign cartilaginous tumor that typically affects the metaphyseal regions of long bones in young individuals. It is characterized by a mixture of cartilage and myxoid tissue, often leading to a well-defined, expansile lytic lesion on radiographs.

**Why the Correct Answer is Right**
The correct answer, Chondromyxoid fibroma, is supported by the radiographic appearance of the knee x-ray in an adolescent boy. The tumor typically presents as a well-defined, lytic lesion with a sclerotic border in the metaphysis of long bones. The mixture of cartilage and myxoid tissue within the tumor leads to a characteristic "soap bubble" appearance on radiographs. In this case, the x-ray shows a well-defined lytic lesion in the metaphysis of the distal femur, which is consistent with the radiographic appearance of Chondromyxoid fibroma.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteosarcoma is a malignant bone tumor that typically presents with a mixed lytic and sclerotic appearance, often with a permeative border and cortical destruction. It is less common in the metaphysis and more likely to affect the epiphysis.

**Option B:** Bone cyst with fracture is a degenerative condition that typically presents as a simple lytic lesion with a well-defined border, often in the metaphysis of long bones. However, the presence of a fracture would be a complicating factor and not the primary diagnosis.

**Option D:** Osteitis Fibrosis cystica is a condition characterized by multiple, well-defined lytic lesions with sclerotic borders, often in the metaphysis of long bones. However, it is typically associated with hyperparathyroidism and is less common in adolescent boys.

**Clinical Pearl / High-Yield Fact**
Chondromyxoid fibroma is a rare, benign tumor that should be considered in the differential diagnosis of well-defined lytic lesions in the metaphysis of long bones in young individuals. It is essential to differentiate it from more aggressive lesions, such as osteosarcoma, and to consider the patient's age and radiographic appearance when making a diagnosis.

**✓ Correct Answer: A. Chondromyxoid fibroma**