Need help identifying this bolt/part

Key details: 2012 Porsche Cayenne -While I was on the freeway today, I heard a knock under my left front wheel well, then violent shaking, loss of braking, basically total control failure. I was fortunately able to slow the car and pull over to the side with no damage other than what is shown below. When I got out, I discovered whatever nut holds that bolt in place departed my vehicle leaving me with a tire/wheel that was uncontrollable. Also, I lost power braking and ABS which is no surprise since the lines disconnected, but those lines appear in ok shape.Can someone help me understand what happened here and what these parts are called? I recently had my front diff disassembled, resealed, and reassembled. Could this be related to a careless reassembly of the front diff?​Thanks all!https://ift.tt/3mWSJ3L