Usher believes a major cause of his poor health is that...
A
he is upset that his only living relative, Madeline, is sick and
may be close to death.
B
the house he lives in is so dark and dirty.
С
his sister Madeline is sick and she probably gave him the
same sickness.
D
a ghost haunts him in his sleep and makes it hard for him to
rest.

Usher believes that one of the main causes of his poor health is that he is sad because his sister, who is the only living relative he has, is sick and may be close to death, as shown in option A.

We can arrive at this answer because:

Usher believes he suffers from a genetic disease that affects everyone in his family.

He already knew that this disease would catch him sooner or later, but after his sister started showing signs of the disease, he realized that he would also be a victim.

His sister is on the verge of death, as the disease has affected her in an imposing way.

This made Usher very sad and afraid, which made his symptoms worse, as he knows he will suffer just like his sister.

This question is about "The Fall of the House of Usher" which tells the story of a house that was dark and decaying as well as its owner's mental state.

You can find more information about this short story here:

https://brainly.com/question/11438422