**Core Concept**
Uniocular diplopia, also known as monocular diplopia, is a symptom where a single eye sees double, often caused by a refractive error or an irregular curvature of the cornea or lens within the eye. In the context of cataract, it is a specific symptom that can help differentiate between the various stages of cataract development.

**Why the Correct Answer is Right**
Uniocular diplopia in cataract is typically seen in the early stages of cataract development, specifically in the nuclear cataract stage. This is because the lens nucleus begins to become more dense and irregular, causing light to be refracted in an irregular manner, resulting in double vision. The denser nucleus of the lens also causes a myopic shift, making the patient more near-sighted.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because posterior subcapsular cataract typically presents with glare and halos at night, not uniocular diplopia.
* **Option B:** This option is incorrect because cortical cataract typically presents with visual disturbances such as halos and glare, but not uniocular diplopia.
* **Option C:** This option is incorrect because mature cataract typically presents with a total loss of vision, not uniocular diplopia.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that uniocular diplopia in cataract is often associated with a denser nucleus, which can also cause a myopic shift. This can be a useful clue for clinicians to differentiate between the various stages of cataract development.

**Correct Answer: A. Nuclear cataract**