Hi Pik Lee,
Get "Category C" as RadPanelItem and check its items. If the count is greater that 1 then this user has access to 2 or 3 child items.If the count equals to 1 then get this child RadPanelItem and make Response.Redirect to it.
// get only the root items - Category A, Category B etc.
(itm.Items.Count == 1)
var child = itm.Items
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