**Core Concept**
Adrenaline (epinephrine) acts on various receptors, including alpha-adrenergic receptors, which mediate vasoconstriction and other effects. Phentolamine is a non-selective alpha-adrenergic receptor antagonist, while propranolol is a beta-adrenergic receptor antagonist.

**Why the Correct Answer is Right**
Phentolamine blocks the effects of adrenaline mediated by alpha-adrenergic receptors, such as vasoconstriction, but propranolol does not. This is because propranolol selectively blocks beta-adrenergic receptors, which are responsible for effects like tachycardia and bronchodilation. Phentolamine, on the other hand, blocks both alpha-1 and alpha-2 adrenergic receptors, preventing the vasoconstrictive effects of adrenaline.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because propranolol would block the effect, not phentolamine.
**Option B:** This is incorrect because phentolamine would block the effect, not propranolol.
**Option C:** This is incorrect because phentolamine would block the effect, not propranolol.
**Option D:** This is incorrect because both phentolamine and propranolol would block the effect.

**Clinical Pearl / High-Yield Fact**
Phentolamine is often used in the treatment of pheochromocytoma, a tumor that secretes excess adrenaline, to control blood pressure.

**Correct Answer: C.**