Which of the following is the shoest acting mydriatic?
**Core Concept**
Tropicamide is a non-selective muscarinic receptor antagonist that induces mydriasis by blocking the muscarinic acetylcholine receptors in the eye, specifically in the ciliary muscle and the iris sphincter muscle. This leads to dilation of the pupil.
**Why the Correct Answer is Right**
Tropicamide has a rapid onset of action, typically within 1-2 minutes, and its effects last for approximately 6-8 hours due to its short duration of action. This is because tropicamide is rapidly hydrolyzed by pseudocholinesterase in the plasma, leading to a quick breakdown of the drug. Its short duration of action makes it an ideal choice for diagnostic purposes, such as before a dilated fundus examination. Tropicamide's fast onset and short duration of action are due to its high lipid solubility, which allows it to easily cross the blood-brain barrier and reach its site of action.
**Why Each Wrong Option is Incorrect**
* **Option A:** Cyclopentolate has a longer duration of action compared to tropicamide, typically lasting 24 hours.
* **Option B:** Atropine has a longer duration of action compared to tropicamide, typically lasting 7-14 days.
* **Option D:** Phenylephrine is a selective alpha-1 adrenergic receptor agonist that causes mydriasis by constricting the radial muscles of the iris, but it is not a mydriatic in the same class as tropicamide.
**Clinical Pearl / High-Yield Fact**
Tropicamide is often used in combination with phenylephrine to achieve both mydriasis and cycloplegia, which is essential for a thorough dilated fundus examination.
**Correct Answer:** C. Tropicamide.