This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
The thyroid inhibitor with fastest mechanism of action is? 
Select and explain. 
A. Potassium iodide, B. Propylthiouracil, C. Carbimazole, D. Cholestyramine
A: Potassium iodide
Answer: A: Potassium iodide 
Explanation: Though iodine is a constituent of the thyroid hormones, but still it is the fastest acting thyroid inhibitor. All the facets of thyroid function seemed to be affected by iodine. But the most impoant function of it is inhibition of the release of the thyroid hormone from the follicular cells which has been already synthesized. Thus, the enlarged gland slowly shrinks, and then, it becomes firm and less vascular.