Force a field to be mandatory if a selection in drop down is selected?

[PPF_Bobbie_Weeks]

Hi all

Is it possible to force a field to become mandatory if a certain selection is made in a drop down list? If the selection isn’t made then the field is there but not mandatory?

For example, there is a drop down choice of A, B, C and D. The field SELECT is always shown but you don’t have to enter anything into it unless you select either choice C or D.

Is this possible and how would I go about it.

Thanks