**Core Concept**
A tumour of the pituitary gland pressing upon the optic chiasma can cause visual defects due to compression of the optic nerves and their fibres as they cross at the optic chiasma. This is a classic example of a structural lesion affecting the visual pathway.

**Why the Correct Answer is Right**
The compression of the optic nerves and their fibres at the optic chiasma results in a specific type of visual defect known as bitemporal hemianopia. This is because the nasal fibres of the optic nerves from each eye cross over at the optic chiasma, and the compression affects these crossing fibres. As a result, the visual field defects are typically seen in the outer (temporal) fields of both eyes.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it describes a condition known as homonymous hemianopia, which is caused by a lesion posterior to the optic chiasma, such as a stroke or tumour in the occipital lobe.
* **Option B:** This option is incorrect because it describes a condition known as central scotoma, which is typically caused by a lesion in the optic nerve or optic disc.
* **Option C:** This option is incorrect because it describes a condition known as quadrantanopia, which is typically caused by a lesion in the temporal lobe or optic radiation.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that visual field defects can be lateralizing, meaning that they can provide clues about the location of a lesion in the brain. For example, a bitemporal hemianopia suggests a lesion in the pituitary gland or optic chiasma, while a homonymous hemianopia suggests a lesion posterior to the optic chiasma.

**Correct Answer: C. Quadrantanopia is incorrect, bitemporal hemianopia is the correct answer.**