Okay, let's tackle this question. The patient is group O, and they received AB blood. He had a transfusion reaction with fever, chills, dyspnea, and hypotension. The question is asking what caused this reaction.

First, I need to recall blood group compatibility. Blood group O is the universal donor for red blood cells because they lack A and B antigens. Recipients with O blood have anti-A and anti-B antibodies in their plasma. So if they receive AB blood, which has both A and B antigens, their antibodies would attack the donor RBCs.

This sounds like an acute hemolytic transfusion reaction. The patient's anti-A and anti-B antibodies would bind to the AB RBCs, causing agglutination and hemolysis. This leads to the symptoms described: fever, chills, hypotension, and possibly renal failure from free hemoglobin.

Now, the options aren't listed, but the correct answer here is likely related to an acute hemolytic reaction. The other options might be other types of reactions like allergic, febrile non-hemolytic, or delayed hemolytic. Let me think why each is wrong. Allergic reactions would present with urticaria, bronchospasm, not hypotension unless anaphylaxis. Febrile reactions are milder, no hemolysis. Delayed reactions happen days later. So the acute hemolytic is the right answer here.

Clinical pearl: Always cross-match blood before transfusion. Group O can only receive O RBCs. AB plasma is universal, but RBCs are different. Remember the antibodies in the recipient's plasma.

**Core Concept**
Acute hemolytic transfusion reactions occur when recipient antibodies attack donor red blood cell (RBC) antigens. Blood group O individuals have anti-A and anti-B antibodies in their plasma, making them incompatible with blood groups A, B, or AB.

**Why the Correct Answer is Right**
The patient (group O) received AB blood, which contains A and B antigens. His anti-A and anti-B IgM antibodies rapidly agglutinate and lyse the donor RBCs via complement activation, causing intravascular hemolysis. This triggers a systemic inflammatory response (fever, chills) and hypotension due to cytokine release and microvascular obstruction by free hemoglobin.

**Why Each Wrong Option is Incorrect**
**Option A:** Allergic reactions present with urticaria, angioedema, or bronchospasm, not hemolysis or hypotension.
**Option B:** Febrile non-hemolytic reactions cause fever and chills but not dramatic hypotension or intravascular hemolysis.
**Option C:** Delayed hemolytic reactions occur 24–72 hours post-transfusion due to IgG-mediated extravascular hemolysis, not acute anaphylaxis.

**Clinical Pearl / High-Yield Fact**
**Group O patients can only receive O RBCs** due to preformed anti-A and anti-B antibodies. AB plasma is universal due to lack of A/B antigens, but RBCs must match. Always confirm ABO compatibility pre-transfusion to prevent acute hemolysis.

**Correct Answer: C. Acute hemolytic transfusion reaction**

✓ Correct Answer: C. Preformed anti-A and anti-B antibodies in the recipient