**Core Concept**
The question is testing the knowledge of the ocular manifestations of AIDS, specifically the most common cause of visual loss in patients with acquired immunodeficiency syndrome. The correct answer is related to the opportunistic infections that affect the retina, leading to severe visual impairment.

**Why the Correct Answer is Right**
Toxoplasmosis is the most common cause of visual loss in AIDS patients. It is caused by the reactivation of latent Toxoplasma gondii infection in the brain. The parasite invades the retina, leading to necrotizing retinitis, which can cause significant visual impairment. The pathophysiology involves the immune system's inability to control the parasite, allowing it to cause damage to the retina.

**Why Each Wrong Option is Incorrect**
* **Option A:** CMV retinitis is another common cause of visual loss in AIDS patients, but it is not the most common cause. CMV retinitis is caused by cytomegalovirus, which also invades the retina, causing necrotizing retinitis.
* **Option B:** Toxocariasis is a parasitic infection caused by Toxocara species, which can affect the eyes, but it is not the most common cause of visual loss in AIDS patients.
* **Option D:** Herpes simplex retinitis can cause visual loss in AIDS patients, but it is not the most common cause. Herpes simplex virus can reactivate in the retina, causing necrotizing retinitis.

**Clinical Pearl / High-Yield Fact**
Toxoplasmosis is a common opportunistic infection in AIDS patients, and it can cause severe visual impairment. Remember that toxoplasmosis is the most common cause of visual loss in AIDS patients, and it often presents with necrotizing retinitis.

**Correct Answer:** C. Toxoplasmosis