Final answer:
An FSH level test is most likely to confirm the cause of the patient's amenorrhea because her symptoms and age suggest she is undergoing menopause, during which FSH levels typically increase.
Step-by-step explanation:
The 53-year-old female patient with symptoms of depression, insomnia, fatigue, hot flashes, and an atrophic vaginal mucosa, along with the fact that her last menstrual period was five months ago, suggests she may be undergoing menopause. Menopause is typically diagnosed clinically, but the most appropriate test to confirm the cause of her amenorrhea would be measuring the Follicle-stimulating hormone (FSH) levels.
During menopause, there is an increase in FSH levels due to a decrease in the hormone inhibin, which normally helps to regulate FSH production through a negative feedback loop. As a woman approaches menopause, the number of viable ovarian follicles decreases, leading to a decrease in estrogen production, and causing menopausal symptoms. Therefore, an elevated FSH level is indicative of menopause, which would confirm the diagnosis in this patient.