Final answer:
Anorexia nervosa is associated with a decrease in prolactin hormone level.
Step-by-step explanation:
Anorexia nervosa is the mental illness associated with a decrease in prolactin hormone level. Prolactin is a hormone produced by the anterior pituitary gland that stimulates milk production. In excess, it can result in symptoms such as amenorrhea, galactorrhea, and breast enlargement. Anorexia nervosa is characterized by severe restriction of food intake, which can lead to hormonal imbalances, including decreased prolactin levels.