Which naturally occurring radioisotope is specifically mentioned as a main source of internal radioactivity within human muscles and bones?

Answer

Potassium-40 ($^{40} ext{K}$)

Potassium-40 is a naturally occurring radioisotope found inside the human body, contributing to the internal component of background radiation dose.

Which naturally occurring radioisotope is specifically mentioned as a main source of internal radioactivity within human muscles and bones?
physicsradiationsourcesemission