Which anticoagulant is found in a green-top tube?

The green-top tube contains heparin as its anticoagulant. Heparin is a more effective anticoagulant when it comes to preventing clotting in blood specimens used for various laboratory analyses. It works by inhibiting thrombin and factor Xa, preventing the formation of fibrin, which is essential for blood clotting.

In contrast, the other anticoagulants mentioned serve different purposes. EDTA, often found in lavender-top tubes, primarily chelates calcium, which is vital for the clotting process. Citrates, commonly in blue-top tubes, also act by binding calcium but are used for coagulation testing. Oxalates, found in gray-top tubes, inhibit clotting by precipitating calcium, similar to EDTA but through a different mechanism. Therefore, heparin's role as the anticoagulant in the green-top tube is specific and essential for the types of tests that require this tube, making it the correct answer.