**Core Concept**
The Arden index is a measure used in **electro-oculography (EOG)** to assess the function of the **retinal pigment epithelium (RPE)**. It is calculated as the ratio of the maximum to minimum **electro-oculogram (EOG) amplitude**. A normal Arden index indicates healthy RPE function.

**Why the Correct Answer is Right**
The correct answer is based on the normal range of the Arden index, which is typically between 175 and 225, but most commonly referenced as being around **185**. This value signifies a normal ratio of light peak to dark trough in the EOG, reflecting adequate **RPE** function and **photoreceptor** health.

**Why Each Wrong Option is Incorrect**
**Option A:** This value is too low, suggesting impaired RPE function.
**Option B:** This is not within the normal range.
**Option D:** This value is too high, not within the standard reference range.

**Clinical Pearl / High-Yield Fact**
Remember, an abnormal Arden index can indicate **Best disease** or other conditions affecting the RPE.

**Correct Answer: C. 185**