**Core Concept**
Coloured halos in a 60-year-old male, splitting and then reuniting on Fincham's test, suggest a specific type of cataract. This phenomenon is related to the interaction between the cataractous lens and the intact crystalline lens, which affects the way light is focused.

**Why the Correct Answer is Right**
The splitting and reuniting of halos on Fincham's test are characteristic of a posterior subcapsular cataract (PSC). In PSC, the opacity is localized to the posterior subcapsular region of the lens, which causes light to be scattered in a way that produces the characteristic splitting and reuniting of halos. This occurs because the PSC cataract acts as a diffracting plate, causing light to bend and create the splitting effect, which then reunites as the light passes through the remaining clear lens tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** Other types of cataracts, such as nuclear sclerotic or cortical cataracts, do not typically produce the characteristic splitting and reuniting of halos on Fincham's test.

**Option B:** The splitting and reuniting of halos on Fincham's test are specific to posterior subcapsular cataracts, not age-related macular degeneration (AMD). AMD may cause visual disturbances, but it does not typically produce the characteristic halos seen in PSC.

**Option C:** The splitting and reuniting of halos on Fincham's test are not characteristic of glaucoma. While glaucoma can cause visual disturbances, the halos seen in PSC are distinct from those seen in glaucoma.

**Clinical Pearl / High-Yield Fact**
A key feature of posterior subcapsular cataracts is their association with diabetes mellitus, which can be a risk factor for the development of PSC.

**Correct Answer: C. Posterior subcapsular cataract.**