## **Core Concept**
Hereditary angioedema (HAE) is a disorder characterized by recurrent episodes of severe swelling, typically affecting the limbs, face, abdomen, and airways. It is primarily caused by a deficiency or dysfunction of the C1 inhibitor (C1-INH), a protein that regulates the complement, coagulation, and kallikrein-kinin systems.

## **Why the Correct Answer is Right**
The correct answer relates to the biochemical changes occurring during an HAE attack. In HAE, the deficiency or dysfunction of C1-INH leads to overactivation of the kallikrein-kinin system, resulting in increased levels of bradykinin, a potent vasodilator that causes increased vascular permeability and edema. Therefore, stating that bradykinin levels decrease during an attack is incorrect.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Dysfunction of the enzyme (C1-INH) is indeed a common cause of HAE, alongside deficiency. This statement is true, making it an incorrect choice as an exception.
- **Option B:** The enzyme involved is indeed C1-INH, making this statement true and not the exception.
- **Option C:** C1 inhibitor targets the Hageman factor (Factor XII) among other things, which is part of its regulatory function. This statement is true.
- **Option D:** Complement C4 & C2 levels do decrease in HAE due to consumption during the activation of the complement system, making this statement true.

## **Why Option E is Correct as the Exception**
- **Option E:** Bradykinin levels actually increase during an HAE attack, not decrease. The increased bradykinin levels are responsible for the increased vascular permeability and edema seen in HAE. Therefore, this option is the exception as it inaccurately describes the change in bradykinin levels during an HAE attack.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that HAE attacks can be life-threatening due to the risk of airway obstruction. Treatment often involves replacement therapy with C1-INH concentrates, ecallantide (a kallikrein inhibitor), or icatibant (a bradykinin receptor antagonist). Knowing that bradykinin levels are elevated during an attack is crucial for understanding the pathophysiology and treatment targets.

## **Correct Answer: E. Bradykinin level decrease during attack**