Phentolamine is a non-selective alpha-antagonist.
Its main action is vasodilation due to alpha-1 blockade.
In the context of MRCP the most important snippet to remember is that phentolamine is used to treat adrenaline-induced ischaemia (eg idiot houseofficer uses lidocaine with adrenaline in ring block, what ya gonna do? or clumsy dentist stabs little finger with his anaesthetic needle and its going blue, how will you treat it?)
In a broader context it can also be helpful in hypertension due to phaeochromocytoma or cocaine.
Thats all for today, more tomorrow!