Help for Javascript !

Hi there ! I am a physican and I try to create a pdf form to request radiology tests. Here is my problem :

I have drop-down lists with some differents types of radiology exams on my form. But for some specific tests, the patients must follow some instructions.

Let's say I select an "abdominal ultrasound" in my drop-down list called "ultrasonography"

I would like to create an "Instruction_field" that displays a specific text IF "abdominal ultrasound" is selected, for example "Drink a lot of water 2 hours before the test"

I know this might seem weird or even obvious for you guys, but I would appreciate your help very much !