Sho acting mydriatics are :
**Core Concept**
Short-acting mydriatics are medications used to dilate the pupils for a short duration, typically in ophthalmic examinations or procedures. They work by relaxing the sphincter pupillae muscle, which is innervated by the parasympathetic nervous system, allowing the dilator pupillae muscle to dominate and cause pupil dilation.
**Why the Correct Answer is Right**
The correct answer is **A. Cyclopentolate**. Cyclopentolate is a non-selective muscarinic receptor antagonist that blocks the action of acetylcholine at the muscarinic receptors in the sphincter pupillae muscle, leading to pupil dilation. This effect is short-acting, lasting around 24 hours, making it suitable for ophthalmic examinations.
**Why Each Wrong Option is Incorrect**
* **Option B:** Atropine is a long-acting antimuscarinic agent that causes prolonged pupil dilation, typically lasting several days.
* **Option C:** Tropicamide is another short-acting mydriatic, but it has a slightly longer duration of action compared to cyclopentolate.
* **Option D:** Phenylephrine is a selective alpha-1 adrenergic receptor agonist that causes vasoconstriction and can cause some mydriasis, but it is not a primary mydriatic agent and is not as effective as cyclopentolate.
**Clinical Pearl / High-Yield Fact**
When using short-acting mydriatics, it's essential to note that they can cause accommodation paralysis, leading to blurred vision. This effect is temporary and usually resolves once the medication wears off.
**Correct Answer: A. Cyclopentolate**