Subgroup typing for A blood type primarily determines the presence of which antigen?

The correct choice is the A1 antigen. Subgroup typing for individuals with blood type A is primarily concerned with distinguishing between the different A subgroups, notably A1 and A2. These two subtypes vary based on the specific antigens present on the red blood cells.

In individuals with blood type A1, the A1 antigen is present in higher quantities compared to those with the A2 antigen. When performing subgroup typing, laboratory tests focus on identifying these nuances because they can have clinical implications, especially in blood transfusions and transplantation situations. The presence of the A1 antigen can affect compatibility, as individuals with A2 blood type may produce anti-A1 antibodies, which could lead to transfusion reactions if an incorrect blood type is administered.

The other options refer to antigens that are not primarily determined by subgroup typing for A blood type. The B antigen and O antigen are unrelated to the specific identification of A blood type subgroups, while the A2 antigen, although relevant, is not the primary focus of subgroup typing, which is mainly aimed at detecting the A1 antigen's presence.