**Core Concept**
The question tests the understanding of the **pupillary dilation** mechanism and the role of **cocaine** in this process. Cocaine blocks the reuptake of **norepinephrine**, a neurotransmitter that stimulates **pupillary dilation**.

**Why the Correct Answer is Right**
Cocaine's failure to dilate the pupil indicates a problem with the **sympathetic nervous system**, which is responsible for pupillary dilation. This could be due to a disruption in the **norepinephrine** pathway.

**Why Each Wrong Option is Incorrect**
**Option A:**
**Option B:**
**Option C:** Horner's syndrome is a condition that affects the sympathetic nervous system, leading to **miosis** (pupillary constriction) and other symptoms.
**Option D:**

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **Horner's syndrome** is characterized by **miosis**, **ptosis**, and **anhydrosis**, and is often associated with **sympathetic nervous system** disruption.

**Correct Answer:** C. Horner's syndrome.