diagnosis, diagnoses

Diagnosis or diagnosis means the determination, usually by the symptoms, of the kind of illness or problem, and where possible its cause --
His diagnosis was correct; she'll need to start treatment for diabetes.
My diagnosis is boredom and fatigue; my prescription is a day trip to the shore.

Diagnoses or diagnoses ( nearly rhymes with diagnosis ) is the he, she, it form of "diagnose", meaning to determine the kind of illness or problem, and where possible its cause --
If the doctor diagnoses a heart condition, he will send you to a specialist.
Our mechanic has a machine that diagnoses engine trouble.

Now that you know that, you can say --
"If the doctor diagnoses psychiatric trouble, you'll be sent to a mental health professional for his  diagnosis."