An ophthalmologist used a mydriatic for dilating the pupils to facilitate ophthalmoscopy. Which of the following is the shoest acting mydriatic?
**Core Concept**
Mydriatics are medications used to dilate the pupils by relaxing the circular muscle of the iris (sphincter pupillae). This allows for better visualization of the retina during ophthalmoscopy. The duration of action of mydriatics can vary, and some are longer-acting than others.
**Why the Correct Answer is Right**
The correct answer is **Cyclopentolate**. Cyclopentolate is a non-selective muscarinic receptor antagonist that blocks the action of acetylcholine on the sphincter pupillae muscle, causing pupil dilation. It has a relatively short duration of action compared to other mydriatics, typically lasting around 24 hours. This is because it is metabolized quickly by the liver and has a low volume of distribution.
**Why Each Wrong Option is Incorrect**
**Option A:** **Tropicamide** is another non-selective muscarinic receptor antagonist, but it has a shorter duration of action than Cyclopentolate, typically lasting around 4-6 hours.
**Option B:** **Atropine** is a long-acting mydriatic that blocks both muscarinic and nicotinic receptors. It has a duration of action that can last up to 2 weeks, making it too long-acting for the description.
**Option D:** **Phenylephrine** is an alpha-adrenergic agonist that causes vasoconstriction and mydriasis, but it is not as effective as muscarinic receptor antagonists like Cyclopentolate and Tropicamide.
**Clinical Pearl / High-Yield Fact**
When choosing a mydriatic, consider the duration of action and the desired effect. Cyclopentolate is a good choice for short procedures or when a quick recovery is desired.
**Correct Answer:** C. Cyclopentolate.