A 42-year-old man with a history of seasonal rhinitis and asthma develops acute anaphylaxis 20 minutes after a bee sting. Serum tryptase is elevated. In contrast, a 38-year-old woman with chronic urticaria and angioedema has normal serum tryptase and responds to antihistamines alone, with no history of acute systemic reactions. Which finding best distinguishes mast cell activation in anaphylaxis from chronic urticaria?

Explanation

## Discriminating Feature: Tryptase Release and Systemic Involvement Both conditions involve **Type I (IgE-mediated) hypersensitivity**, but they differ in **magnitude of mast cell degranulation** and **systemic manifestations**. ### Anaphylaxis **Key Point:** Acute, life-threatening systemic reaction with rapid mast cell degranulation and release of large amounts of preformed mediators. - Onset: minutes (typically <30 min) - Elevated serum tryptase (reflects mast cell degranulation) - Systemic symptoms: hypotension, syncope, bronchospasm, laryngeal edema - Cardiovascular collapse possible - Requires immediate epinephrine ### Chronic Urticaria **Key Point:** Localized or mild systemic IgE-mediated reaction with limited mast cell activation; normal or low tryptase levels. - Onset: variable, often insidious - Normal serum tryptase (minimal degranulation) - Localized symptoms: urticaria, angioedema, pruritus - No systemic hypotension or respiratory compromise - Responds to antihistamines and corticosteroids ### Comparison Table | Feature | Anaphylaxis | Chronic Urticaria | | --- | --- | --- | | **Onset** | Minutes | Hours to days | | **Serum tryptase** | Elevated (>11.4 ng/mL) | Normal (<11.4 ng/mL) | | **Mast cell degranulation** | Massive, acute | Minimal, chronic | | **Systemic involvement** | Hypotension, syncope, shock | Localized skin/mucosal | | **Respiratory symptoms** | Bronchospasm, laryngeal edema | Absent | | **Treatment** | Epinephrine (first-line) | Antihistamines, corticosteroids | | **Prognosis** | Life-threatening | Chronic but non-fatal | **High-Yield:** **Serum tryptase is the biochemical marker of acute, massive mast cell degranulation.** Elevated tryptase (>11.4 ng/mL) in the context of acute systemic symptoms is pathognomonic for anaphylaxis. Chronic urticaria, despite being IgE-mediated, does not trigger sufficient mast cell degranulation to elevate tryptase. **Mnemonic:** **TRYPTASE = ANAPHYLAXIS** — if serum tryptase is elevated during an acute reaction, think anaphylaxis, not simple urticaria. [cite:Harrison 21e Ch 333] **Clinical Pearl:** Tryptase should be drawn within 15 minutes to 3 hours of symptom onset in suspected anaphylaxis. A normal tryptase does not exclude anaphylaxis if drawn too late, but elevated tryptase is highly specific for acute mast cell activation. [cite:Robbins 10e Ch 6] --- ## Why Option 0 is Correct Elevated serum tryptase is a biochemical hallmark of acute, massive mast cell degranulation and is the single best discriminator between anaphylaxis (acute, systemic, life-threatening) and chronic urticaria (localized, manageable). The presence of systemic involvement (hypotension, shock) further confirms anaphylaxis.