**Core Concept**
Warfarin and heparin are two types of anticoagulants used to prevent blood clot formation. Warfarin works by inhibiting the production of vitamin K-dependent clotting factors in the liver, while heparin directly inhibits thrombin and factor Xa. In this scenario, the patient is on heparin, which requires reversal to prevent excessive bleeding.

**Why the Correct Answer is Right**
The patient's elevated aPTT and retroperitoneal hematoma indicate that sheparin has caused excessive anticoagulation. Protamine sulfate is a specific antidote for heparin, as it directly binds to heparin and forms a stable complex that neutralizes its anticoagulant activity. This rapid reversal is essential in cases of heparin-induced bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** Fresh frozen plasma (FFP) contains all clotting factors and can correct coagulopathy, but it is not the specific antidote for heparin and may not be as effective or rapid in reversal.

**Option B:** Vitamin K is used to reverse warfarin's effects by promoting the synthesis of vitamin K-dependent clotting factors. However, it is not indicated in this case, as warfarin is not the primary anticoagulant.

**Option C:** Desmopressin is used to treat bleeding disorders such as hemophilia A and von Willebrand disease by releasing von Willebrand factor and factor VIII. It is not indicated in this case of heparin-induced bleeding.

**Option D:** Recombinant factor VIIa is used to treat bleeding in patients with coagulopathy, including those with liver disease or trauma. However, it is not the specific antidote for heparin and may not be as effective or safe in this scenario.

**Clinical Pearl / High-Yield Fact**
When managing patients on heparin, it's essential to remember that protamine sulfate is the specific antidote and should be administered promptly in cases of heparin-induced bleeding.

**Correct Answer:** C. Protamine sulfate.