Factual errors: If Sarah's blood-type is O-negative, then it is impossible for her to have a son that is AB-negative. Assuming that Kyle was AB-negative like Derek, then John could only be A- or B-negative; not even O-negative. Also, either John or Sarah should have been able to give Derek her blood, as she is a near universal donor, Derek is a near universal acceptor and they all have the same Rh factor.
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