GMAT Critical Reasoning (CR) | Re: A study comparing a group of chronically depressed

A study comparing a group of chronically depressed individuals with an otherwise matched group of individuals free from depression found significantly more disorders of the immune system among the depressed group. According to the researchers, these results strongly support the hypothesis that mental states influence the body’s vulnerability to infection.
Which of the following, if true, casts the most serious doubt on the researchers’ interpretation of their findings?
(A) The researchers’