**Core Concept**
Blood transfusion reactions can occur due to ABO incompatibility between the donor and recipient blood types, leading to hemolysis and potentially life-threatening complications. This requires immediate recognition and management to prevent further harm.

**Why the Correct Answer is Right**
The first step in managing a suspected blood transfusion reaction is to **stop the transfusion** immediately. This is crucial in preventing further hemolysis and reducing the risk of acute kidney injury, cardiac instability, and even death. The transfusion should be halted as soon as the reaction is suspected, and the recipient should be closely monitored for signs of hemolysis, such as jaundice, hemoglobinuria, or renal failure.

**Why Each Wrong Option is Incorrect**
* **Option A:** Administering more blood is incorrect because it may worsen the reaction and lead to further hemolysis.
* **Option B:** Administering hydrocortisone is incorrect because while corticosteroids may be used to manage allergic reactions, they do not address the underlying issue of ABO incompatibility.
* **Option D:** Administering methylene blue is incorrect because it is used to treat methemoglobinemia, which is a different type of blood disorder.

**Clinical Pearl / High-Yield Fact**
In a blood transfusion reaction, the first thing to remember is to "Stop, Stop, Stop" - stop the transfusion immediately, stop the infusion of any additional blood products, and stop the administration of any other potentially incompatible solutions.

**Correct Answer:** C. Stop the transfusion.