Please can someone assist with the following?

At present I have a data validation list in Excel (which is working fine).
However, when a user selects one of the options in the list I need Excel to
show a popup with a warning message (as this option in the list works
differently to the others). How can I make Excel do this? I can make a popup
in Excel using datavalidation but since this cell already has this (for the
list) how can I replicate again?

Any help gratefully received.

TIA.