This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
The source of endogenous radiation is - 
Select and explain. 
A. Radon, B. Potassium, C. Thorium, D. Uranium
B: Potassium
Answer: B: Potassium 
Explanation: Park's textbook of preventive and social medicine 23rd edition. * Man is also subjected to internal radiation,i.e, from radioactive matter stored in the body tissues, it includes minute quantities of uranium , thorium, isotopes of potassium,strontium and carbon.