**Core Concept**
Mydriatics are medications used to dilate the pupils, which is essential in ophthalmic examinations, such as tonometry and fundoscopy. The duration of action of mydriatics varies, with some causing long-lasting dilation and others providing only temporary effects.

**Why the Correct Answer is Right**
Tropicamide is a short-acting anticholinergic mydriatic that works by inhibiting the muscarinic receptors in the iris sphincter muscle. This leads to relaxation of the sphincter muscle and subsequent dilation of the pupil. Tropicamide has a rapid onset of action and a relatively short duration of effect, typically lasting around 4-6 hours.

**Why Each Wrong Option is Incorrect**
**Option A:** Atropine is a long-acting mydriatic that can cause dilation of the pupils for up to 7-14 days. It is not the shortest acting mydriatic.
**Option B:** Cyclopentolate is a medium-acting mydriatic that can cause dilation of the pupils for around 24 hours. It is not as short-acting as tropicamide.
**Option C:** Phenylephrine is a sympathomimetic mydriatic that can cause dilation of the pupils, but it is not as effective or commonly used as tropicamide.

**Clinical Pearl / High-Yield Fact**
Remember that tropicamide is often used as a diagnostic agent to assess the red reflex in infants and young children, which can help diagnose conditions such as congenital cataracts.

**Correct Answer: C. Tropicamide**