Which radioactive element is most commonly detected naturally inside the human body?

Introduction / Context:
This question examines basic knowledge of natural radioactivity in living organisms. Even without exposure to man made radiation sources, humans contain small amounts of naturally occurring radioactive isotopes. Understanding which of these isotopes is most common in the human body helps connect nuclear physics to biology and health science, and it commonly appears in general science sections of competitive exams.

Given Data / Assumptions:

• We are asked which radioactive element is most commonly detected in humans under normal conditions.
• Options include potassium 40, cobalt 60, iodine 131, plutonium 238, and None of the above.
• We assume ordinary, healthy individuals without unusual contamination or medical treatment.

Concept / Approach:
Potassium is an essential element for life, present in all cells. A small fraction of natural potassium consists of the radioactive isotope potassium 40 (K 40). Because our bodies contain significant potassium, we naturally contain K 40 as well. This makes potassium 40 the most common radioactive element in humans. Cobalt 60 and iodine 131 are used in medical treatments and diagnostics but are not naturally present in large amounts. Plutonium 238 is an artificial transuranic element not normally found in the body.

Step-by-Step Solution:
Step 1: Recall that potassium is a major intracellular ion; all humans contain several tens of grams of potassium. Step 2: Recognise that natural potassium includes a small percentage of K 40, which is radioactive. Step 3: Therefore, anyone with normal potassium levels also carries K 40 in their tissues. Step 4: Compare with cobalt 60 and iodine 131, which are manufactured isotopes used in radiotherapy and imaging and do not accumulate naturally in healthy individuals. Step 5: Conclude that potassium 40 is the naturally occurring radioactive element most commonly detected in humans.

Verification / Alternative check:
Background radiation studies often mention that a significant portion of internal radiation exposure comes from K 40 inside the body. Another contributor is carbon 14, but it is not listed among the options here. Cobalt 60 and iodine 131 appear mainly in the context of cancer therapy and thyroid imaging, respectively, and decay away over time after treatment. Plutonium 238 is related to nuclear power and space power sources, not to normal human biochemistry. These comparisons confirm that K 40 is the correct answer.

Why Other Options Are Wrong:
Cobalt-60: A man made isotope used for radiotherapy; it is not naturally abundant in the human body. Iodine-131: Used in thyroid diagnosis and treatment; present only temporarily and not in all humans. Plutonium-238: A synthetic element used in specialised applications; not normally found in biological tissues. None of the above: Incorrect because potassium 40 is correctly listed and is the natural internal source of radiation in humans.

Common Pitfalls:
Students may be distracted by isotopes like iodine 131 and cobalt 60 because they are frequently mentioned in medical contexts. However, these are artificial and episodic in the body, not constant background sources. To answer correctly, remember that potassium and carbon are fundamental elements in biology, and their natural radioactive isotopes (K 40 and C 14) are the main contributors to internal radioactivity under normal conditions.

Final Answer:
The radioactive element most commonly detected naturally in humans is potassium-40.