Sarah has major depressive disorder. the medication she takes to treat her symptoms likely increases the availability of in her brain

The medication will increase the availability of serotonin in the brain.

Explanation:

Serotonin is a neurotransmitter and hormone related mainly to one's mood. It is well known as the happiness hormone, so the more serotonin released in the brain, the more likely the person is to be in a happy mood.

A person with the mayor depressive disorder has problems releasing serotonin into the brain, which is one of the cause of depression.

In neurobiology, once a neurotransmitter is released into the brain, it is later recaptured. An antidepressant blocks this area to allow the serotonin to flow freely in the brain for a longer period, thus helping the person fight the depression and generally being in a better mood.