%HP Physical Damage
This is the reason that there needs to be more than just Zhonya's for AP champions to defend against AD. Bork and now Bloddrazer both grant this, in addition to being increased 50% by Guinsoo's, so having more health via Rylai, RoA, Hextech, or Liandry's doesn't effectively defend against it.
And yes, tank items do exist. But if the argument is that Randuin's can be bought on everyone, then Maw should just be removed and AD champs forced to purchase Visage or Banshee.
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are the ones I remember right now) and the reason is the one I explained before, maybe one item wouldn't change it too much, but it would either be a must buy or something that's never a good buy, even if the situation needs it.