**Core Concept**
Uniocular diplopia, also known as monocular double vision, is a condition where a patient sees two images when looking through one eye. This phenomenon occurs due to irregularities in the optical system of the affected eye, including misalignment of the extraocular muscles, cataracts, or other refractive errors.

**Why the Correct Answer is Right**
The commonest cause of uniocular diplopia is a cataract, particularly a posterior subcapsular cataract (PSC). This type of cataract affects the posterior part of the lens, causing light to be scattered and leading to double vision. The presence of a cataract disrupts the normal optical pathway, resulting in the perception of two images. In contrast, other causes of uniocular diplopia, such as strabismus or keratoconus, are less common.

**Why Each Wrong Option is Incorrect**
**Option A:** Strabismus, or misalignment of the eyes, typically causes binocular diplopia, not uniocular diplopia. While strabismus can cause double vision, it affects both eyes simultaneously.
**Option B:** Keratoconus, a progressive thinning of the cornea, can cause irregular astigmatism and visual distortion, but it is not the most common cause of uniocular diplopia.
**Option C:** Retinal detachment can cause uniocular diplopia, but it is a less common cause compared to cataracts.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with uniocular diplopia, it is essential to consider the patient's visual history, including any previous eye surgeries, trauma, or systemic diseases that may be contributing to the condition.

**Correct Answer: C. Retinal detachment**