All of the following causes coloured halos except
**Core Concept**
Coloured halos are a symptom often associated with corneal or lenticular abnormalities, resulting from the scattering of light as it passes through the affected tissue. This phenomenon is typically observed as a concentric ring of color around light sources, such as when looking at a light bulb or the sun.
**Why the Correct Answer is Right**
Retinal degeneration does not cause coloured halos because it primarily affects the photoreceptors and retinal pigment epithelium, leading to visual loss and other symptoms like night blindness or peripheral vision loss. The scattering of light, which is responsible for the formation of coloured halos, occurs in the anterior segment of the eye, particularly in the cornea or lens, and is not directly related to retinal degeneration.
**Why Each Wrong Option is Incorrect**
**Option A:** Corneal edema can cause coloured halos due to the swelling of corneal tissue, which leads to the scattering of light as it passes through the cornea.
**Option C:** A foreign body in the cornea can cause coloured halos as light is scattered by the irregular surface of the cornea.
**Option D:** Steroids, particularly in the form of topical eye drops, can cause posterior subcapsular cataracts, which can lead to the formation of coloured halos.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that coloured halos are often associated with conditions affecting the anterior segment of the eye, such as corneal edema, cataracts, or foreign bodies. In contrast, retinal degeneration is more likely to cause other symptoms like visual loss or night blindness.
**✓ Correct Answer: B. Retinal degeneration**