**Core Concept**
Optic atrophy refers to the degeneration of the optic nerve, which can result from various causes such as glaucoma, ischemia, or compression. The clinical manifestation of optic atrophy includes pallor of the optic disc, which is a critical diagnostic feature.

**Why the Correct Answer is Right**
Pallor of the optic disc in optic atrophy is primarily due to the loss of retinal ganglion cells and their axons, which leads to a decrease in the number of melanin-containing cells in the optic disc. This reduction in melanin-containing cells results in a pale appearance of the optic disc. The pallor is often accompanied by other signs such as optic cup enlargement and thinning of the nerve fiber layer.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the clinical manifestation of optic atrophy.
* **Option B:** This option is incorrect because optic atrophy is not primarily characterized by swelling of the optic disc.
* **Option C:** This option is incorrect because optic atrophy is not primarily characterized by increased intraocular pressure.

**Clinical Pearl / High-Yield Fact**
It's essential to note that optic disc pallor can also be seen in conditions such as optic neuritis, where there is inflammation of the optic nerve. A careful examination of the patient's history and other clinical features is necessary to differentiate between these conditions.

**Correct Answer:** D.