**Core Concept**
The Ardenone's sign is a clinical indicator used to differentiate between afferent pupillary defects (APD) and efferent pupillary defects (EPD). In APD, the affected pupil dilates when a light source is moved from the normal eye to the affected eye due to impaired afferent input from the affected eye.

**Why the Correct Answer is Right**
When the flashlight is moved from the normal eye to the abnormal eye, the affected pupil dilates due to the relative afferent pupillary defect. This occurs because the affected eye's retina or optic nerve has impaired function, resulting in a reduced or absent afferent signal to the Edinger-Westphal nucleus. The Edinger-Westphal nucleus then sends a reduced or absent parasympathetic signal to the pupil, causing it to dilate.

**Why Each Wrong Option is Incorrect**
**Option A:** Adie's tonic pupil would show a sustained pupillary constriction rather than dilation in response to light.
**Option B:** Horner's syndrome would present with miosis (pupil constriction) due to impaired sympathetic input, not dilation.
**Option C:** Syringomyelia would lead to a different set of clinical manifestations, including loss of pain and temperature sensation, and might not specifically cause a relative afferent pupillary defect.

**Clinical Pearl / High-Yield Fact**
Remember that the Ardenone's sign is a key indicator of afferent pupillary defects, such as those caused by optic nerve damage or retinal disease. This clinical finding can be used to differentiate between afferent and efferent pupillary defects.

**Correct Answer:** C. Syringomyelia