Which of the following diagnoses would most likely present with difficulty in vision changes and redness in the eye?

Iritis, also known as uveitis, refers to the inflammation of the iris, which is the colored part of the eye. This condition can lead to significant symptoms such as changes in vision, often described as blurriness or disturbances, and redness in the eye due to associated inflammation.

In cases of iritis, patients may also experience pain, photophobia (sensitivity to light), and sometimes an irregularly shaped pupil. The redness typically arises not just from surface irritation, but from deeper structures within the eye becoming inflamed. Hence, the combination of vision changes and redness strongly points towards iritis.

The other conditions mentioned typically present with different symptoms or do not primarily cause significant changes in vision alongside redness. For instance, viral and bacterial conjunctivitis often come with increased discharge, itching, and more mild changes in vision compared to iritis. Allergic conjunctivitis may present with redness and itching as well, but it does not usually induce significant changes in vision. Therefore, iritis stands out as the most likely diagnosis in this scenario.