**Question:** The cause of a coloured &Halo; in narrow angle glaucoma is

A. Iris constriction
B. Iris recession
C. Iris thickening
D. Increased aqueous humor production

**Correct Answer:** D. Increased aqueous humor production

**Core Concept:** In narrow angle glaucoma, the key event is the narrowing of the angle between the cornea and the iris, leading to increased intraocular pressure (IOP) and damage to the optic nerve.

**Why the Correct Answer is Right:** The coloured halo or "halo sign" is a characteristic symptom seen in patients with narrow angle glaucoma due to the increased aqueous humor production. Aqueous humor is the fluid that fills the space between the cornea and the iris called the anterior chamber. In normal circumstances, this fluid is produced by the ciliary body and flows out through the trabecular meshwork into the anterior chamber angle, which is the space between the cornea, iris, and the Schlemm's canal. In narrow angle glaucoma, the angle between the cornea and iris is very narrow, causing a reduction in the outflow of aqueous humor. This results in increased intraocular pressure (IOP) and subsequent optic nerve damage, which leads to characteristic optic disc changes and visual field defects.

**Why Each Wrong Option is Incorrect:**

A. Iris constriction: Iris constriction would lead to a decrease in the angle width, exacerbating the narrow angle rather than causing the halo sign.
B. Iris recession: Similar to option A, this would also worsen the narrow angle, not cause the halo sign.
C. Iris thickening: Iris thickening would result in a narrower angle, but it is not the cause of the halo sign.

**Clinical Pearl:** The halo sign or the coloured ring around the pupil is a crucial sign in suspecting narrow angle glaucoma. Timely diagnosis and treatment can prevent severe vision loss and optic nerve damage.

**Why the Correct Answer is Right:** The correct answer, increased aqueous humor production, causes a narrow angle and increased IOP, which leads to the halo sign in narrow angle glaucoma. This answer highlights the crucial role of aqueous humor production and its impact on the angle width and intraocular pressure.

**Why Each Wrong Option is Incorrect:**

A. Iris constriction: Iris constriction would worsen the narrow angle, not cause the halo sign.
B. Iris recession: Similar to option A, this would also worsen the narrow angle, not cause the halo sign.
C. Iris thickening: Iris thickening would result in a narrower angle, but it is not the cause of the halo sign.

**Clinical Pearl:** The halo sign in narrow angle glaucoma is a crucial clue for diagnosing this potentially devastating disease. Timely diagnosis and treatment can prevent severe vision loss and optic nerve damage.