If radio button is selected, text field required-Acrobat form

Hi everyone!

I'm creating a form in Adobe Acrobat form 9.0, and I ran into a little problem.

There's a section in my form that has four options.  If one option is selected, the textbox to the right is required.

I don't really know how to do that.  If anyone know about to do this, please help.  I'd really appreciate it.

FYI, I'm not using Adobe LiveCycle.

Thank you for your time!