What is the antidote for heparin overdose?

Protamine sulfate is the specific antidote for heparin overdose, making it the correct choice in this scenario. Heparin is an anticoagulant that works by inhibiting factors in the clotting cascade, which can lead to excessive bleeding if overdosed. Protamine sulfate acts as a heparin antagonist by binding to heparin and neutralizing its anticoagulant effect. This binding forms a stable complex that no longer possesses anticoagulant properties, thereby reversing the effects of heparin quickly.

In contrast, other options do not counteract heparin's effects. Calcium carbonate is primarily an antacid and does not play a role in managing anticoagulant toxicity. Vitamin K is used as an antidote for warfarin toxicity rather than heparin, making it inappropriate in this context. Activated charcoal may be useful in certain poisonings but is not effective for heparin overdose due to its inadequate absorption and the rapid action of heparin in the system. Therefore, protamine sulfate is the established treatment for heparin overdose, directly reversing its action and potentially preventing serious complications associated with excessive anticoagulation.