Final answer:
Roderick Usher's deteriorating state is linked to an acute illness, a mental disorder, and a superstitious belief in the dark influence of his family mansion.
Step-by-step explanation:
After the narrator arrives, Roderick Usher reveals that his present state stems largely from his acute bodily illness, a mental disorder that oppresses him, and his superstitious beliefs regarding the influence of his family mansion on his spirit. The house's gloomy and deteriorating physical state is mirrored by Usher's own declining mental and physical health. Roderick's appearance and the aura of the house suggest a former robustness and vitality that have long been fading away, which is reflected in the narrator's observations of Usher's changed features and the vegetation overgrowing the mansion's exterior. Additionally, his nervous agitation and trepidancy further compound his dismal condition, as described by the narrator's recollections and the disconcerting letter that summoned him to Usher's side.