Bitemporal hemianopsia results from a lesion at which site?

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Bitemporal hemianopsia occurs when the crossing fibers at the optic chiasm are damaged, especially in the central, medial portion where the nasal retinal fibers from both eyes cross. The nasal retina carries signals for the temporal (outer) visual fields, and these fibers cross at the chiasm; a lesion there disrupts the temporal fields of both eyes, producing loss in the outer halves bilaterally. If the damage were before the chiasm, vision would be lost in one eye; if after the chiasm, you’d get a homonymous field loss affecting the same side in both eyes. The lateral geniculate nucleus is downstream (post-chiasm) and its lesions usually cause homonymous deficits rather than bitemporal ones.