A patient with severe anemia becomes cyanotic and develops tachycardia after receiving blood without fever. What type of reaction is this most likely?

The situation described involves a patient experiencing cyanosis and tachycardia after receiving a blood transfusion, without the presence of fever. These symptoms are indicative of Transfusion-Associated Circulatory Overload (TACO), which occurs when there is an overload of volume or an excessively rapid transfusion of blood products, leading to heart and respiratory distress.

In this case, severe anemia could exacerbate the heart's response to increased blood volume, causing the heart to work harder and resulting in elevated heart rates (tachycardia) and possible impaired oxygenation (cyanosis). Unlike febrile reactions, which are accompanied by fever and chills due to immune responses, or hemolytic reactions, which often present with hemoglobinemia, fever, and back pain due to immune-mediated destruction of red blood cells, TACO is primarily concerned with volume overload and does not typically involve febrile symptoms. Anaphylactic reactions can occur but usually present with more severe allergic symptoms, such as hives, angioedema, or respiratory distress, rather than the cardiovascular and cyanotic issues observed here.

Thus, the combination of symptoms and the context of the transfusion indicates that TACO is the most likely diagnosis in this scenario.