Conditional validation using entity framework

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ASP.NET

Given the entity, I want to be able to alter the validation for Choice depending on if NotApplicable is checked.

public class Answer
{
    public bool NotApplicable { get; set; }
    [Range(1, 5, ErrorMessage = "Choice must be 1-5")]
    public int Choice { get; set; }
}

Form:

@using (Html.BeginForm()) 
{
    @Html.AntiForgeryToken()
    
    <div class="form-horizontal">
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })

        <div class="form-group">
            @Html.LabelFor(model => model.NotApplicable,
                htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                <div class="checkbox">
                    @Html.EditorFor(model => model.NotApplicable)
                    @Html.ValidationMessageFor(model => model.NotApplicable,
                        "", new { @class = "text-danger" })
                </div>
            </div>
        </div>

        <div class="form-group">
            @Html.LabelFor(model => model.Choice, htmlAttributes:
                new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @for (Int32 opt = 1; opt < 7; opt++)
                {
                    <div>
                        @Html.Label(opt.ToString())
                        @Html.RadioButtonFor(model => model.Choice, opt)
                    </div>
                }
                @Html.ValidationMessageFor(model => model.Choice,
                    "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Create" class="btn btn-default" />
            </div>
        </div>
    </div>
}

What I have tried:

I have tried removing the validation and doing it all on the submit, but that seems a bit stupid. I would really like if the use clicks N/A then the choice is blank and disabled.
Any ideas?