I need to create several validation rules off the back of the same process - we have a picklist and we need to enforce that if A, B or C are selected from A, B, C, D or E, that another field on the page becomes mandatory. I cannot find any code anywhere to create this and am struggling with the CASE and ISPICKVAL functions. Can anybody help? Thanks so much,

I need to create several validation rules off the back of the same process - we have a picklist and we need to enforce that if A, B or C are selected from A, B, C, D or E, that another field on the page becomes mandatory. I cannot find any code anywhere to create this and am struggling with the CASE and ISPICKVAL functions. Can anybody help? Thanks so much,