Which uncommon differential of neovascular AMD primarily affects patients under the age of 50?

Idiopathic choroidal neovascularization (ICN) is indeed the correct choice as an uncommon differential of neovascular age-related macular degeneration (AMD) that primarily affects patients under the age of 50. ICN is characterized by the emergence of choroidal neovascular membranes without any associated systemic or ocular disease. It typically occurs in younger patients, often with no identifiable risk factors or pre-existing conditions that might lead to neovascularization. Thus, its incidence in this age group sets it apart from other conditions listed, making it a key consideration in differential diagnoses for younger individuals with vision loss.

In contrast, polypoidal choroidal vasculopathy is more commonly seen in older patients, can resemble AMD but is often associated with different demographics and risk factors. Ocular histoplasmosis syndrome usually affects middle-aged individuals and is associated with a history of exposure to histoplasmosis, while age-related macular degeneration primarily occurs in older adults, typically over 50 years of age. Therefore, the distinct age demographic associated with idiopathic choroidal neovascularization solidifies its role in this question, highlighting its significance as an uncommon but relevant diagnosis in younger patients.