A 6-year-old male presents with hemarthrosis of the left knee and prolonged aPTT. What is the best treatment option for this patient?

Desmopressin acetate is the best treatment option for this patient presenting with hemarthrosis and prolonged activated partial thromboplastin time (aPTT). The scenario described is suggestive of a bleeding disorder, which commonly includes hemophilia, particularly Hemophilia A, characterized by a deficiency in factor VIII.

Desmopressin acetate works by stimulating the release of von Willebrand factor and factor VIII from endothelial cells, thereby increasing their levels in the circulation. This is particularly effective in patients with mild hemophilia or when the bleeding is not severe, as it can rapidly correct the aPTT and help control bleeding episodes such as hemarthrosis in the knee.

Other treatment options listed are not as suitable for this specific clinical situation. Corticosteroids might be employed for inflammatory conditions or other conditions affecting joints, vitamin K is used primarily for deficiencies related to vitamin K-dependent clotting factors, and cryoprecipitate, which contains fibrinogen and factor VIII, is typically reserved for more immediate or severe bleeding scenarios when specific factor replacement therapy is not available.

Desmopressin is, therefore, the most appropriate choice to manage this child's bleeding episode effectively.