**Core Concept**
The body contains small amounts of naturally occurring radioactive substances, primarily due to the presence of certain isotopes in the body's tissues. These isotopes are formed through the decay of radioactive elements or the ingestion of radioactive materials.

**Why the Correct Answer is Right**
Potassium-40 (40K) is a naturally occurring radioactive isotope of potassium, which is present in the body's tissues. It is formed through the decay of uranium-238 and thorium-232 in the body. 40K decays into argon-40 and calcium-40, with a half-life of approximately 1.25 billion years. This means that the body continuously loses and replaces potassium-40 through radioactive decay, which is a normal process.

**Why Each Wrong Option is Incorrect**
**Option A:** Carbon-14 (14C) is a radioactive isotope of carbon, but it is not naturally present in the body in significant quantities. It is formed in the atmosphere through nuclear reactions and is incorporated into the body through the food chain.

**Option B:** Radium-226 (226Ra) is a radioactive isotope of radium, but it is not naturally present in the body. It is a byproduct of uranium decay and can accumulate in the body through exposure to contaminated water or food.

**Option C:** Technetium-99m (99mTc) is a radioactive isotope of technetium, but it is not naturally present in the body. It is a synthetic isotope used in nuclear medicine for imaging and diagnostic purposes.

**Clinical Pearl / High-Yield Fact**
The body's natural radiation exposure comes from three main sources: cosmic radiation from space, radon gas in the home, and internal radiation from naturally occurring isotopes like potassium-40.

**Correct Answer: C. Potassium-40**