<s>[INST] context: There are two types of vessels in coronary circulaton –

1. Larger conducting arteries which run epicardially and send perforating branches.
2. Smaller resistance arterioles - These are perforating branches of conducting arteries. These arterioles supply blood to the endocardium. 

 During ischemia, adenosine (a local mediator) causes dilatation of resistance arterioles in the ischemic area only, the tone of resistance vessels of the nonischemic area does not change → the increased flow to the ischemic area.
 Dipyridamole is a powerful coronary dilator; increases total coronary flow by preventing uptake and degradation of adenosine.
 It dilates resistance vessels in the nonischemic zone as well → Diverts the already reduced blood flow away from ischemic zone - Coronary steal phenomenon.
 It has no effect on larger conducting vessels (contrast nitrates which dilate larger conducting vessels and cause redistribution of blood towards ischemic zone).
 Dipyridamole inhibits platelet aggregation by increasing cAMP (by inhibiting phosphodiesterase) which potentiates PGI and interferes with aggregation.
 It is not useful as an antianginal drug (due to coronary steal phenomenon), but can be used for prophylaxis of coronary and cerebral thrombosis in post-MI and post-stroke patients as well as to prevent thrombosis in patients with a prosthetic heart valve. [/INST] question: Coronary steal phenomenon is seen with: | options: A: Dipyridamole B: Diltiazem C: Propranolol D: Verapamil | answer: A </s>