## **Core Concept**
Posterior uveitis refers to inflammation of the posterior segment of the eye, which includes the vitreous, retina, and choroid. Infectious causes of posterior uveitis can be due to various pathogens, including bacteria, viruses, fungi, and parasites. The commonest infectious cause often varies geographically and depends on the population being studied.

## **Why the Correct Answer is Right**
Toxoplasmosis, caused by the protozoan *Toxoplasma gondii*, is widely recognized as the most common cause of posterior uveitis, particularly in immunocompetent individuals. The infection often results in retinochoroiditis, which is characterized by necrosis of the retina and choroid. Reactivation of latent toxoplasmosis can lead to the formation of new lesions, which are typically located near old scars.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain bacteria can cause posterior uveitis, they are not as commonly associated with this condition as *Toxoplasma gondii*.
- **Option B:** Viral infections can cause posterior uveitis, but they are less commonly identified as the cause compared to toxoplasmosis.
- **Option C:** Fungal infections are a significant cause of posterior uveitis, especially in immunocompromised patients or those with a history of fungal exposure, but they are not the most common cause overall.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that toxoplasmosis typically presents with a "headlight in the fog" appearance on fundoscopy, which is a well-defined, rounded area of retinochoroiditis often with a lighter center and surrounding pigmentation. This appearance is classic but not pathognomonic. Serological tests can support the diagnosis, but they indicate exposure rather than active disease.

## **Correct Answer:** D. Toxoplasmosis.