Final answer:
In an AIDS patient with decreased visual acuity, the most likely offending infectious agent is Cytomegalovirus (CMV), which can cause severe retinitis in immunocompromised individuals.
Step-by-step explanation:
An immunocompromised AIDS patient presenting with decreased visual acuity is most likely experiencing an infection caused by Cytomegalovirus (CMV), also known as human herpesvirus 5 (HHV-5). CMV is a common infection in the general population but can become a serious health concern in patients with compromised immune systems, such as those with AIDS. While the majority of CMV infections are asymptomatic in healthy individuals, the virus can cause severe and potentially vision-threatening retinitis in AIDS patients. CMV retinitis is one of the opportunistic infections that may develop as the CD4 T-cell count decreases significantly in the progression of AIDS.