PICO is a mnemonic used to describe the four elements of a good clinical question. It stands for:

P - Patient/Problem/Population
I - Intervention
C - Comparison
O - Outcome

For Diagnosis, you may consider using:

P - Patient
I - Test
C - Standard
O - Outcome

For Prognosis:

P- Patient
F - Factors
O - Outcome (ex. Mortality)

Here are links for more information.