A 72-year-old woman from Mumbai with a history of hypertension and diabetes presents with progressive bilateral visual field loss over 3 weeks. She reports difficulty reading and bumping into objects on both sides. On perimetry, there is a bitemporal hemianopia with superior temporal quadrantanopia on the left and inferior temporal quadrantanopia on the right. MRI brain with contrast shows a 2 cm sellar mass compressing the optic chiasm from below. What is the most likely diagnosis?