Which medication should be given to a casualty in shock who has just received one unit of blood?

In the context of managing a casualty in shock who has just received one unit of blood, administering calcium is important due to its role in the clotting process and in stabilizing cardiac function. When blood products are transfused, especially in situations involving massive transfusion, there is a risk of hypocalcemia due to the citrate anticoagulant used during blood storage. Citrate binds calcium, potentially leading to a decrease in ionized calcium levels which can result in complications such as cardiac arrhythmias, impaired contractility, and coagulopathy.

By administering calcium, healthcare providers can help mitigate these risks, ensuring that the casualty's heart functions optimally and that the coagulation cascade remains effective. This becomes especially critical in a trauma setting where maintaining hemostatic function is crucial to patient outcomes.

In contrast, the other options—Vitamin K, Magnesium, and Sodium—are not indicated for immediate treatment in this specific scenario. Vitamin K may be used in the context of managing vitamin K antagonist overdoses or in certain bleeding situations but is not typically a first-line response to shock or blood transfusion. Magnesium does play roles in various physiological processes, but is not directly related to the urgent care required for a casualty in shock post-transfusion. Sodium is