**Core Concept**
The diurnal variation of intraocular pressure (IOP) refers to the fluctuations in IOP that occur throughout the day, influenced by various physiological and pathological factors. This variation is a critical aspect of glaucoma management, as elevated IOP is a major risk factor for glaucomatous optic neuropathy.

**Why the Correct Answer is Right**
The IOP typically follows a circadian rhythm, peaking in the early morning hours and decreasing to a nadir in the late evening. This diurnal fluctuation is thought to be mediated by the autonomic nervous system, with sympathetic activity contributing to increased IOP during wakefulness. The peak IOP in the morning is often referred to as the "morning spike," and it is during this period that glaucoma patients are at greatest risk of optic nerve damage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the typical pattern of diurnal IOP variation.
**Option B:** This option is incorrect because it is not supported by scientific evidence; the diurnal variation of IOP is not as pronounced in the evening as it is in the morning.
**Option C:** This option is incorrect because it is not a recognized characteristic of diurnal IOP variation.

**Clinical Pearl / High-Yield Fact**
It is essential for glaucoma patients to have their IOP measured in the morning, as this is when the pressure is typically highest and most likely to cause damage to the optic nerve.

**Correct Answer: C. There is no IOP variation.**