Final answer:
Epinephrine is the drug that treats heart block, cardiac arrest, and anaphylactic shock and serves as a vasoconstrictor in local anesthetics. Beta-blockers like metoprolol and propranolol affect the heart differently by blocking beta receptors.
Step-by-step explanation:
The correct answer to the question is A. Epinephrine. Epinephrine is used to treat severe allergic reactions known as anaphylactic shock, and it also treats heart block and cardiac arrest. Additionally, epinephrine is a vasoconstrictor that can be added to local anesthetic preparations to prolong the effects of the anesthetic. On the other hand, beta-blockers, such as metoprolol and propranolol, work in part by blocking beta receptors in the heart, which can reduce heart rate and blood pressure and are commonly used in the treatment of cardiovascular diseases.