**Core Concept**
The patient's presentation of pain on movement, combined with a lytic lesion on the X-ray of the knee joint, suggests a bone tumor or cyst affecting the upper end of the tibia. This condition may be related to a metabolic disorder, a benign growth, or a malignant process.

**Why the Correct Answer is Right**
The presence of a lytic lesion in the upper end of the tibia is characteristic of osteochondritis dissecans (OCD). OCD occurs when a piece of cartilage and bone becomes partially or fully detached from the surrounding bone due to inadequate blood supply. This condition often affects the weight-bearing joints, such as the knee, and is more common in young adults. The patient's pain on movement is likely related to the mechanical instability caused by the detached cartilage and bone fragment.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteosarcoma is a malignant bone tumor that typically presents with a mixed lytic and sclerotic appearance on X-ray, not a purely lytic lesion. Additionally, osteosarcoma typically affects the metaphysis of long bones, not the epiphysis.
**Option B:** Giant cell tumor of bone is a benign tumor that affects the epiphysis of long bones, but it is more common in older adults and typically presents with a mixed lytic and sclerotic appearance on X-ray.
**Option C:** Bone cysts, such as unicameral bone cysts, can present as lytic lesions, but they typically affect the metaphysis of long bones, not the epiphysis.

**Clinical Pearl / High-Yield Fact**
Osteochondritis dissecans is a common cause of knee pain in young adults, and the presence of a lytic lesion on X-ray can be a key diagnostic clue. It's essential to consider the patient's age, activity level, and joint involvement when evaluating this condition.

**Correct Answer:** C.