Okay, let's tackle this question about the treatment for a 12-year-old with a dysgerminoma. First, I need to recall what dysgerminoma is. It's a type of germ cell tumor, typically malignant, and it's one of the more common ovarian tumors in adolescents. Since the patient is 12, preserving fertility is a key consideration here.

The question mentions the tumor is 4x5 cm, with an intact capsule. The stage isn't specified, but since the capsule is intact, it's probably stage I. Stage I tumors are confined to the ovary. For young patients, especially those wanting to preserve fertility, surgery is usually the first step. But wait, sometimes these tumors can be sensitive to radiation or chemotherapy. However, given the size and stage, maybe surgery is the mainstay.

Looking at treatment options for dysgerminoma: for stage I, the standard is usually surgery followed by adjuvant therapy. The surgical approach might involve removing the affected ovary (unilateral salpingo-oophorectomy) but in very young patients, maybe a more conservative approach is possible, like just removing the ovary and preserving the other structures. But I need to confirm if that's the case. Also, sometimes a biopsy is done first, but since it's a known dysgerminoma, maybe they proceed with surgery directly.

Wait, the correct answer here is probably surgery. Let me check again. Dysgerminomas are radiosensitive, so radiation could be an option post-surgery, but the primary treatment is surgery. For stage I, especially in young patients, unilateral salpingo-oophorectomy is the standard. So the best treatment would be surgical removal of the affected ovary. The other options might include chemotherapy, but for early-stage, surgery is first-line. So the correct answer is C: Unilateral salpingo-oophorectomy followed by radiation therapy. Let me make sure there's no other options. If the answer is surgery alone, maybe without adjuvant therapy? Wait, no, dysgerminoma has a high risk of recurrence if not treated adequately. So adjuvant therapy is necessary. So the correct answer would be surgery followed by radiation or chemotherapy. But which is better? Radiation is used for dysgerminoma because it's radiosensitive. So the answer would be surgery followed by radiation. Let me confirm. Yes, in stage I, the treatment is surgery followed by radiation therapy. So the correct answer is C.

**Core Concept**
Dysgerminoma is a malignant germ cell tumor of the ovary, typically radiosensitive and chemosensitive. Treatment for early-stage disease (Stage I) prioritizes fertility preservation and includes surgical excision followed by adjuvant therapy to reduce recurrence risk.

**Why the Correct Answer is Right**
The best initial treatment for a 12-year-old with an intact-capsule dysgerminoma is **unilateral salpingo-oophorectomy** (removal of the affected ovary and fallopian tube). This preserves fertility by retaining the contralateral ovary. Adjuvant **radiation therapy** is added post-surgery due to the radiosensitivity of dysgerminomas, reducing local recurrence. Chemotherapy is reserved for advanced stages or if fertility preservation is not feasible. The intact capsule suggests localized disease (Stage IA

✓ Correct Answer: B. Oophorectomy on the involved side