How can I use DropDownListFor() on a dynamic type?

I get the following error in the DropDownListFor() "An expression tree may not contain a dynamic operation" because the lambda uses a dynamic type.

How can I set the selected option on the DropDownList without resorting to jQuery?

I would also rather not make a template or custom helper.

Model

public class Thing : Base {
    public virtual Nullable<int> OptionID { get; set; }
    public virtual Option Option { get; set; }
}
public class Option : Base {
    public virtual ICollection<Thing> Things { get; set; }
}
public class Base {
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
}

Controller

public ActionResult Edit(int Id) {
    return View(new ViewModel(context, new Thing()));
}

View

@model MvcApp7.Models.ViewModel
@{
    var Entity = (dynamic)Model.Entity;
}

@Html.DropDownListFor(x => Entity.OptionID , (System.Web.Mvc.SelectList)Model.Options)

ViewModel

public class ViewModel {
    public ViewModel(Context context, object entity) {
        this.Entity = entity;
        this.Options = new SelectList(context.Options, "Id", "Name");
    }
    public dynamic Entity { get; set; }
    public SelectList Options { get; set; }
}

EDIT: Please excuse me. I forgot that I could specify the selected option in the SelectList itself. I moved the responsibility into the ViewModel and will try to deal with it from there. However, it would still be good to know how to work around this in the View itself in case it was necessary.

I did this in the ViewModel
this.Options = new SelectList(context.Options, "Id", "Name", this.Entity.OptionID);
and this in the View
@Html.DropDownList("OptionID", Model.Options)