Final answer:
Platelet transfusion is contraindicated for TTP and HUS because it can worsen abnormal clotting in small blood vessels.
Step-by-step explanation:
Platelet transfusion is contraindicated for Thrombotic Thrombocytopenic Purpura (TTP) and Hemolytic Uremic Syndrome (HUS) because it can exacerbate the conditions and lead to severe complications. TTP and HUS are both characterized by abnormal clotting in small blood vessels.
In TTP, the presence of ultra-large von Willebrand factor (vWF) multimers causes platelets to form microthrombi within blood vessels. Platelet transfusion can further contribute to clotting and worsen the condition.
In HUS, the underlying cause is often an infection, such as Shiga toxin-producing Escherichia coli. The infection leads to endothelial damage and platelet activation, resulting in clot formation. Platelet transfusion can increase the number of platelets available for activation and exacerbate clotting.