**Core Concept**
Radiological findings in leukaemia can indicate bone marrow infiltration, which affects bone density and structure. Leukaemia can cause various changes in bone X-rays, including osteolytic lesions, osteoporosis, and periosteal reactions.

**Why the Correct Answer is Right**
Leukaemia can cause osteolytic lesions, especially in flat bones, due to bone marrow infiltration by leukaemic cells. This leads to the destruction of bone tissue, resulting in lytic lesions on X-rays. In children, leukaemia often affects the metaphysis, causing metaphyseal osteoporosis due to bone marrow infiltration. Periosteal new bone formation can occur in response to leukaemic infiltration, but it's not the most characteristic finding in leukaemia. Osteosclerosis, or increased bone density, is not typically associated with leukaemia. Transverse lines of dark bands below the growth plate, also known as "leukaemic lines," are a characteristic radiological finding in leukaemia, indicating periods of rapid bone growth.

**Why Each Wrong Option is Incorrect**
* **Option A:** Osteolytic lesions in flat bones are a characteristic finding in leukaemia, but the other options are not as specific or characteristic.
* **Option B:** Metaphyseal osteoporosis is a feature of leukaemia, but it's not as distinctive or specific as the correct answer.
* **Option C:** Periosteal new bone formation can occur in response to leukaemia, but it's not a typical or specific finding in this condition.
* **Option D:** Osteosclerosis is not typically associated with leukaemia, as it's more often seen in conditions with increased bone turnover.

**Clinical Pearl / High-Yield Fact**
Leukaemic lines, or transverse lines of dark bands below the growth plate, are a characteristic radiological finding in leukaemia, indicating periods of rapid bone growth. This finding is often seen in children with leukaemia and can be a clue to the diagnosis.

**Correct Answer:** e. Transverse line of dark band below the growth plate.