The table shows the results of blood type tests on samples from four individuals. Which row represents an individual who can accept transfusions from people with blood type AB?

[Source: © International Baccalaureate Organization 2019]

B

This was the most debated question among the examining team and no doubt also in schools. It was another question where the majority of candidates chose one incorrect answer, in this case B. The blood of that individual clumped with both Anti-A and Anti-B serum so must have been in blood group AB. They were therefore able to accept blood of any type.

The individual we are looking for cannot accept blood of group A or AB. This suggests that they are in group B and answer A is therefore correct, as that individual’s blood clotted with Anti-B serum. Ironically this was the least popular answer.

However, we are not told in the question whether the person could accept blood of group B. If they could not, they would have been in blood group O rather than B, giving answer C. This was the second least popular answer! Because of this ambiguity in the question, both answers A and C were accepted. Even then, less than 20 % of candidates got one of the allowed answers and the discrimination index was very low.

Questions on blood groups are usually too easy, so it is something of a rarity to have constructed such a difficult question. It was based on this statement in 11.1 of the AHL: ‘Antigens on the surface of red blood cells stimulate antibody production in a person with a different blood group’ but it proved to be much too complicated.