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Anaphylaxis is a severe, life-threatening allergic reaction that occurs rapidly after exposure to an allergen. The pathophysiology involves multiple systems and responses in the body.

The primary events in anaphylaxis include bronchospasm and vasodilation, which result from the release of mediators such as histamine from mast cells and basophils. Histamine causes smooth muscle contraction in the bronchi, leading to bronchospasm, which results in difficulty breathing. It also causes vasodilation, which contributes to decreased systemic vascular resistance and can lead to hypotension.

Increased vascular permeability is another critical aspect of anaphylaxis. Histamine and other mediators increase the permeability of blood vessels, allowing fluids and proteins to leak into the surrounding tissues. This effect can lead to third spacing, where fluid accumulates in interstitial spaces, leading to significant swelling and contributing to decreased blood volume and cardiovascular instability.

While hemolysis and disseminated intravascular coagulopathy are not typically associated with anaphylaxis, they can occur in specific severe cases or other conditions but are not core features of anaphylaxis itself. However, the correct answer encapsulates the primary and well-understood mechanisms involved in anaphylaxis, which are bronchosp