A 22-year-old woman presents with blurred vision and floaters. Fundoscopy shows a yellow-white focal necrotising retinochoroidal lesion with dense overlying vitreous haze, located immediately adjacent to a heavily pigmented chorioretinal scar. Serology is positive for Toxoplasma gondii IgG. What is the underlying mechanism of this presentation?

• A Haematogenous spread of newly acquired systemic infection seeding the macula
• B Reactivation of dormant encysted organisms at the edge of an old scar ✓
• C Direct extension of infection from the optic nerve head
• D Autoimmune reaction to retinal S-antigen without viable organisms

Explanation

Ocular toxoplasmosis is the commonest cause of posterior uveitis worldwide, and recurrences arise from reactivation of dormant tissue cysts lying at the margins of an old pigmented scar, producing the classic headlight in the fog appearance of a bright necrotising lesion behind vitreous haze. Positive IgG reflects prior exposure and supports reactivation. Haematogenous spread describes primary acquisition, which is less typical of this scar-adjacent presentation, and direct extension from the nerve head is not the recognised route.

Reference: Parsons' Diseases of the Eye, 23rd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.