How to enable the field that is multiple line of text?

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In a form, I initialize an item list. In this list i have columns : question, response and description. In some questions I have only response called '(n/a). While the n/a answer is applicable I want to enable the description column. However, I cannot use the enable on choice field. I have tried to put condtions in a 'column edit restricion' but this does not work as it would require add another response and that is not what buisness would like.