What is the most common cause of Neonatal alloimmune thrombocytopenia (NAIT)?

Neonatal alloimmune thrombocytopenia (NAIT) primarily occurs due to the incompatibility between maternal and fetal platelet antigens. The most common cause of NAIT is maternal antibodies directed against paternal antigens on the fetal platelets. Among the various human platelet antigens (HPAs), HPA-1a is the most frequently implicated in cases of NAIT.

In this condition, if a mother is HPA-1a negative and the fetus inherits the HPA-1a antigen from the father, the mother may produce antibodies against this antigen. These antibodies can cross the placenta and lead to a low platelet count in the fetus, resulting in NAIT.

Understanding the prevalence of different HPA antibodies in the context of NAIT underscores why HPA-1a is the correct answer when identifying the most common cause of this condition. The other HPAs, while they can be involved in NAIT, do not have the same frequency of association as HPA-1a.