**Core Concept**
The diurnal variation of intraocular pressure (IOP) is an essential factor in the diagnosis of glaucoma. Glaucoma is a group of eye conditions that damage the optic nerve, often due to abnormally high pressure in the eye. The diurnal variation of IOP refers to the normal fluctuations in IOP that occur throughout the day.

**Why the Correct Answer is Right**
The American Academy of Ophthalmology recommends that a diurnal variation of IOP greater than 6 mmHg can be used as a diagnostic criterion for glaucoma. This is because a significant diurnal variation in IOP can indicate underlying problems with the eye's drainage system or increased resistance to outflow, which can lead to elevated IOP and optic nerve damage. The diurnal variation is typically measured by taking IOP readings at multiple times throughout the day, usually at waking and sleeping times.

**Why Each Wrong Option is Incorrect**
* **Option A:** A diurnal variation of IOP greater than 3 mmHg is not typically considered significant for glaucoma diagnosis. While some fluctuation in IOP is normal, this amount is within the expected range and does not necessarily indicate glaucoma.
* **Option B:** A diurnal variation of IOP greater than 9 mmHg is not a commonly used diagnostic criterion for glaucoma. While it may indicate a problem with the eye's drainage system, it is not a specific or sensitive diagnostic marker.
* **Option C:** A diurnal variation of IOP greater than 4 mmHg is not a widely accepted diagnostic criterion for glaucoma. While some studies have suggested that this level of fluctuation may be associated with glaucoma, it is not a standard diagnostic measure.

**Clinical Pearl / High-Yield Fact**
Remember that the diurnal variation of IOP is just one factor in the diagnosis of glaucoma. A comprehensive eye examination, including visual field testing and optic disc evaluation, is essential for accurate diagnosis and management.

**Correct Answer:** C.