**Core Concept**
In pediatric ophthalmology, mydriatics are used to dilate the pupils for refraction in infants. The goal is to allow for a more accurate measurement of the refractive error by reducing the effect of the lens on the light entering the eye.

**Why the Correct Answer is Right**
The correct answer is **Cyclopentolate**. It is an anticholinergic mydriatic agent that works by blocking the muscarinic receptors in the iris sphincter muscle, leading to pupillary dilation. Cyclopentolate is the preferred agent in infants due to its rapid onset and short duration of action, making it ideal for refraction studies.

**Why Each Wrong Option is Incorrect**
**Option A:** Atropine is a longer-acting mydriatic agent that can cause significant systemic side effects, making it less suitable for infants. It is often used in adults and children for dilating the pupils for examination.
**Option B:** Phenylephrine is a selective alpha-1 adrenergic receptor agonist that causes vasoconstriction and mydriasis. However, it is not typically used for refraction in infants due to its potential to increase intraocular pressure.
**Option D:** Tropicamide is another anticholinergic mydriatic agent that is commonly used in adults and children. However, it has a slower onset and shorter duration of action compared to cyclopentolate, making it less ideal for refraction studies in infants.

**Clinical Pearl / High-Yield Fact**
When using mydriatics in infants, it is essential to perform a thorough examination of the eyes to rule out any pre-existing conditions that may be exacerbated by the dilation.

**Correct Answer: C. Cyclopentolate**