**Core Concept**
Langerhans cell histiocytosis (LCH) is a rare disorder characterized by the proliferation of abnormal Langerhans cells, leading to various clinical manifestations. It can affect multiple organs and tissues, including the bone, skin, liver, and lungs.

**Why the Correct Answer is Right**
Gonadal involvement in Langerhans cell histiocytosis is extremely rare and not a common feature of the disease. The typical presentation of LCH includes involvement of the bone, skin, and hematopoietic system, but gonadal involvement is not a characteristic or common finding. This is because LCH is primarily a disorder of the immune system, and the gonads are not typically affected.

**Why Each Wrong Option is Incorrect**
**Option A:** The peak incidence of Langerhans cell histiocytosis is indeed less than 3 years of age, making this statement true. LCH often presents in early childhood, with most cases diagnosed in children under the age of 3.
**Option B:** Langerhans cell histiocytosis is indeed radiosensitive, meaning that radiation therapy can be an effective treatment option for certain cases of LCH. This is particularly true for patients with localized disease or those who are not candidates for chemotherapy.
**Option C:** The diffuse form of Langerhans cell histiocytosis is known as Letterer-Siwe disease, which is a true statement. Letterer-Siwe disease is characterized by a diffuse and widespread involvement of LCH, often affecting multiple organs and tissues.

**Clinical Pearl / High-Yield Fact**
A key thing to remember about Langerhans cell histiocytosis is that it can mimic other conditions, such as malignancies or autoimmune disorders. Therefore, a high index of suspicion and a thorough diagnostic workup are essential for accurate diagnosis and treatment.

**✓ Correct Answer: D. Gonadal involvement occurs**