This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: A.N.S., Subject name: Pharmacology
Drug S is most like 
Select and explain. 
A. Epinephrine, B. Isoproterenol, C. Norepinephrine, D. Phenylephrine
B: Isoproterenol
Answer: B: Isoproterenol 
Explanation: The effects of Drug S are changed by treatment with the beta blocker, but not by the alpha blocker (choices A, C, and D are ruled out). Terbutaline is beta2 selective and would not increase heart rate directly. Drug S is isoproterenol.