**Question:** Immediately after eating, a man develops swelling of face and lips, respiratory distress, intense pruritis, hypotension and feeling of impending doom. The most likely diagnosis is –

A. Anaphylaxis
B. Allergic rhinitis
C. Angioedema
D. Agranulocytosis

**Correct Answer:** A. Anaphylaxis

**Core Concept:** Anaphylaxis is a life-threatening systemic hypersensitivity reaction that occurs in response to exposure to an allergen, usually through the ingestion of a food or drug, or the injection of a venom. It involves the activation of mast cells and basophils, leading to the release of histamine and other mediators which cause vasodilation, increased vascular permeability, and activation of smooth muscle cells.

**Why the Correct Answer is Right:** Anaphylaxis shares overlapping features with other allergic reactions, such as allergic rhinitis (B) and angioedema (C), but it presents with a more severe and systemic involvement. Angioedema (option C) is a localized swelling of the skin, mucous membranes, or gastrointestinal tract, while anaphylaxis (option A) involves not only skin and mucosa but also causes systemic symptoms like hypotension, respiratory distress, and impending doom. Additionally, anaphylaxis is characterized by a rapid onset and progression, unlike the slow progression of angioedema.

**Why Each Wrong Option is Incorrect:**

A. Angioedema (C): While angioedema can affect the skin and mucous membranes, it typically does not present with hypotension, respiratory distress, or a rapid progression.

B. Allergic rhinitis (B): This condition affects the nasal passages and respiratory tract, causing symptoms like sneezing, rhinorrhea, and nasal congestion, but does not involve hypotension or systemic symptoms.

D. Agranulocytosis (D): Agranulocytosis is a rare condition characterized by a decrease in granulocytes, mainly neutrophils, leading to increased susceptibility to infections. It is not related to food or drug ingestion and presents with fever, chills, and severe infection symptoms, not the rapid onset symptoms of anaphylaxis.

**Clinical Pearls:**

Understanding the rapid onset and systemic involvement of anaphylaxis is crucial in differentiating it from other conditions like angioedema, allergic rhinitis, and agranulocytosis. Prompt recognition and management of anaphylaxis is necessary to prevent severe complications and mortality. Prompt administration of epinephrine, antihistamines, and corticosteroids is essential for controlling the reaction and stabilizing the patient's condition.