**Core Concept**
Radioactive elements have been used in medical practice for various diagnostic and therapeutic purposes, but some have fallen out of favor due to safety concerns, availability of alternative treatments, or advancements in technology.

**Why the Correct Answer is Right**
Radium-226 (Ra-226) was once used in medicine, particularly in the form of radium-containing paints, for the treatment of various conditions, including ankylosing spondylitis and certain types of cancer. However, its use has been largely discontinued due to the high risk of radiation exposure and the availability of safer alternatives. The primary concern with Ra-226 is its high energy beta particles, which can cause significant tissue damage and increase the risk of radiation-induced cancers.

**Why Each Wrong Option is Incorrect**
**Option A:** Iodine-131 (I-131) is still widely used in nuclear medicine for the treatment of thyroid disorders, such as hyperthyroidism and thyroid cancer. It is not a discontinued radioactive element.
**Option B:** Technetium-99m (Tc-99m) is a popular radiopharmaceutical used in nuclear medicine for diagnostic imaging procedures. It is not a radioactive element that has fallen out of favor.
**Option C:** Cobalt-60 (Co-60) is still used in radiation therapy for the treatment of certain cancers, including brain tumors and certain types of lymphoma. It is not a discontinued radioactive element.

**Clinical Pearl / High-Yield Fact**
Radium-226 was once used in medicine, but its use has been largely discontinued due to safety concerns and the availability of safer alternatives.

**Correct Answer:** D. Radium-226.