Final answer:
The sphenoid sinus is the paranasal sinus most proximal to the adenohypophysis, located just anterior and inferior to the sella turcica.
Step-by-step explanation:
The paranasal sinus most proximal to the adenohypophysis (or the anterior pituitary gland) is the sphenoid sinus. The sphenoid sinus is a single, midline sinus located within the body of the sphenoid bone, just anterior and inferior to the sella turcica, where the pituitary gland resides. This makes the sphenoid sinus the most posterior of the paranasal sinuses and the one most closely related to the adenohypophysis.