Highlight row on drop down selection

Maybe you can attach a copy of your spreadsheet and I will try to apply the format for you.

Note: I have assumed your dropdown list is a result of Data Validation. If it is a combobox instead, then you will need to assign the cell beneath the combobox... and then the Conditional formatting will be =$A$1=3 where 3 represents the third entry in your list which would correspond to "high"