A 30-year-old man develops generalized urticaria, angioedema, and bronchospasm within minutes of receiving a local anaesthetic for a laceration repair. The agent used was an ester-type local anaesthetic. Which of the following best explains the pathogenesis of this allergic reaction?

• A Direct histamine release from mast cells due to the preservative methylparaben.
• B Formation of para-aminobenzoic acid (PABA) metabolites that act as haptens. ✓
• C High plasma concentration of the parent drug causing direct complement activation.
• D Inhibition of plasma pseudocholinesterase leading to accumulation of the active drug.

Explanation

Ester local anaesthetics are metabolized to para-aminobenzoic acid (PABA), which acts as a hapten and triggers true IgE-mediated allergic reactions. Amides are not metabolized to PABA and rarely cause true allergies. While methylparaben can cause idiosyncratic reactions, PABA is the classic mechanism for ester hypersensitivity. Pseudocholinesterase deficiency causes toxicity from succinylcholine or esters, not an allergic response.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.