**Core Concept**
Bitemporal hemianopic field defect is a type of visual field defect characterized by the loss of vision in the outer half of each visual field. This condition arises due to damage or compression of the optic chiasm, which is the site of crossing of the optic nerves from each eye.

**Why the Correct Answer is Right**
A pituitary tumour, particularly a non-functioning pituitary adenoma, can compress the optic chiasm from below, leading to bitemporal hemianopia. This occurs because the pituitary gland is located near the optic chiasm and a growing tumour can exert pressure on the chiasm, disrupting the crossing of the optic nerves and resulting in a characteristic loss of vision in the outer half of each visual field. The optic nerves from each eye carry information from the nasal half of the retina, and when these nerves are compressed, the visual information from the temporal half of the visual field is lost.

**Why Each Wrong Option is Incorrect**
**Option A:** Glaucoma typically causes peripheral field defects, such as arcuate scotomas, rather than bitemporal hemianopia.
**Option B:** Optic neuritis causes unilateral visual loss, usually affecting one eye, and is characterized by pain on eye movement and decreased visual acuity.
**Option D:** Retinal detachment causes a sudden onset of visual loss, usually affecting one eye, and is characterized by a "curtain" of vision loss, rather than bitemporal hemianopia.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with a bitemporal hemianopic field defect, it is essential to consider a pituitary tumour as a potential underlying cause, particularly if the patient presents with symptoms of endocrine dysfunction or a history of headaches or visual disturbances.

**✓ Correct Answer: C. Pituitary tumour**