**Core Concept**
The use of radioisotopes in systemic radionuclide therapy involves the administration of radioactive materials that target specific tissues or organs to treat various diseases, including certain types of cancer. Systemic radionuclide therapy delivers the radioactive material throughout the body, allowing for the treatment of systemic diseases.

**Why the Correct Answer is Right**
The correct answer is **A. I-131**. Iodine-131 is a radioactive isotope used in the treatment of thyroid cancer, as it selectively targets thyroid tissue and emits beta and gamma radiation, leading to cell death. The mechanism of action involves the uptake of I-131 by the thyroid gland, which is then concentrated in the thyroid tissue, allowing for targeted radiation therapy.

**Why Each Wrong Option is Incorrect**
**Option B:** Technetium-99m is a radioactive isotope used in nuclear medicine for diagnostic imaging, not systemic radionuclide therapy. It is primarily used for imaging various organs and tissues, such as the bone, lung, and liver.

**Option C:** Samarium-153 is a radioactive isotope used in systemic radionuclide therapy for the treatment of bone metastases. It is selectively taken up by bone tissue, making it an effective treatment for bone cancer.

**Clinical Pearl / High-Yield Fact**
When using radioisotopes in systemic radionuclide therapy, it is essential to consider the specific targeting and distribution of the radioactive material within the body to minimize side effects and maximize therapeutic efficacy.

**Correct Answer: A. Iodine-131**