**Core Concept**
The Arden index is a measure used in electro-oculography (EOG) to assess the function of the retinal pigment epithelium. It is calculated as the ratio of the maximum to minimum standing potential of the eye. The **retinal pigment epithelium** plays a crucial role in maintaining the visual cycle and overall eye health.

**Why the Correct Answer is Right**
The normal value of the Arden index is typically considered to be around 185. This value indicates healthy function of the retinal pigment epithelium, with normal ratios of maximum to minimum standing potential. The **EOG** measures the corneo-retinal standing potential, which is generated by the metabolic activity of the retinal pigment epithelium.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the normal range of the Arden index.
**Option B:** This option is also incorrect as it is not a recognized normal value for the Arden index.
**Option D:** This option is incorrect because it falls outside the established normal range for the Arden index.

**Clinical Pearl / High-Yield Fact**
The Arden index is an important diagnostic tool in ophthalmology, helping to identify diseases affecting the retinal pigment epithelium, such as **Best vitelliform macular dystrophy**. A reduced Arden index may indicate impaired retinal function.

**Correct Answer:** D. 185