A patient's lips swell dramatically immediately after rubber dam removal at the end of root canal treatment. The patient has no prior history of this reaction. What is the most likely cause?

The sudden onset of lip and facial swelling immediately upon removal of the rubber dam strongly suggests angioedema from a latex (natural rubber latex) hypersensitivity reaction. Latex allergy can manifest as localized urticaria, angioedema, or in severe cases, anaphylaxis. The rubber dam is a common source of latex allergen exposure in dentistry. Angioedema of the lips, tongue, and throat can be life-threatening and requires immediate management: epinephrine (0.3 mg IM), antihistamines, corticosteroids, and airway monitoring. All future treatment for this patient should use latex-free materials.