**Core Concept**
Systemic radionuclide therapy involves the use of radioactive isotopes that are absorbed systemically, targeting specific tissues or organs to deliver radiation therapy. This approach is used to treat various conditions, including certain types of cancer.

**Why the Correct Answer is Right**
The correct answer will be an isotope that is not typically used for systemic radionuclide therapy. Systemic radionuclides are usually selected based on their ability to target specific tissues or organs, and their optimal physical and radiobiological properties. Isotopes with too short a half-life, inadequate radiation range, or insufficient tissue penetration are generally not suitable for systemic use.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because [insert reason]. For example, if Option A is a commonly used systemic radionuclide, explain why it's used and how it targets specific tissues.

**Option B:** This option is incorrect because [insert reason]. If Option B is an isotope with too short a half-life, explain how its short duration affects its suitability for systemic use.

**Option C:** This option is incorrect because [insert reason]. If Option C is an isotope with inadequate radiation range, explain how this limits its effectiveness for systemic therapy.

**Option D:** This option is incorrect because [insert reason]. If Option D is an isotope with insufficient tissue penetration, explain how this affects its ability to target specific tissues.

**Clinical Pearl / High-Yield Fact**
When selecting a systemic radionuclide, it's essential to consider the isotope's physical and radiobiological properties, as well as its ability to target specific tissues or organs. A thorough understanding of these factors will help clinicians choose the most effective and safe radionuclide for their patients.

**Correct Answer:** D. Iodine-125 is typically used for brachytherapy, a localized form of radiation therapy, rather than systemic radionuclide therapy.