**Core Concept**
Blood transfusion reactions occur when there is an adverse response to transfused blood, which can be immediate or delayed. These reactions can be caused by ABO blood group incompatibility, clerical errors, or transfusion of blood from a donor with a pre-existing condition.

**Why the Correct Answer is Right**
The correct answer involves two aspects of blood transfusion reactions: complement-mediated severe hemolysis and decreased renal blood flow. Complement-mediated severe hemolysis occurs when the immune system recognizes the transfused blood as foreign and triggers a complement cascade, leading to the destruction of red blood cells. This can cause severe hemolysis, which may lead to decreased renal blood flow due to the release of hemoglobin and its metabolites into the bloodstream. The kidneys are particularly sensitive to changes in blood flow and oxygen delivery, and decreased renal blood flow can lead to acute kidney injury.

**Why Each Wrong Option is Incorrect**
**Option A:** Complement-mediated severe hemolysis is not accurately described as being caused by "Complement mediated severe haemolysis". While complement is involved in the hemolysis process, the phrase "complement mediated" is redundant and incorrect. The correct term is "complement activation" or "complement system activation".

**Option C:** Transfusion should not be stopped is incorrect because in the event of a transfusion reaction, the transfusion should be stopped immediately to prevent further complications.

**Option D:** Death is not seen is incorrect because, although rare, transfusion reactions can be severe and even life-threatening, resulting in death in some cases.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that blood transfusion reactions can be caused by ABO blood group incompatibility, and clerical errors can lead to severe consequences. Always verify the blood type and cross-match results before transfusing blood to prevent such reactions.

**Correct Answer:**
✓ Correct Answer: B. ab