Where there is proof of the drug sale, proof of the death or serious bodily injury, and circumstantial evidence that the two were related, a rational finder of fact might conclude that the sale was of the drugs that caused the death or serious bodily injury. Same if victim testifies that they were "pretty sure" that they bought the drugs in question from the deft.
Where one deft actually enters into the transaction and another leaves the drugs in the WC for the customer, there is sufficient evidence for the finder of fact to conclude that the drugs were purchased from the latter.