A 62-year-old man with hypertension is started on enalapril. After 3 weeks, he develops a persistent dry cough. The cough resolves completely after switching to losartan. Which of the following statements about this adverse drug reaction is NOT true?

## Classification of ACE Inhibitor-Induced Cough ### Type A vs Type B Adverse Reactions **Key Point:** ACE inhibitor-induced cough is a **Type B (bizarre/idiosyncratic) reaction**, not Type A. This is a high-yield distinction in ADR classification. | Feature | Type A (Augmented) | Type B (Bizarre) | |---------|-------------------|------------------| | **Mechanism** | Exaggeration of known pharmacological action | Unpredictable, unrelated to primary intended action | | **Dose-dependent** | Yes, increases with dose | No, independent of dose | | **Predictability** | Predictable from pharmacology | Idiosyncratic, occurs in susceptible individuals | | **Incidence** | Common | Relatively uncommon (5–20% of ACE inhibitor users) | | **Example** | Beta-blocker bradycardia, hypoglycaemia with insulin | ACE inhibitor cough, penicillin anaphylaxis | ### Why ACE Inhibitor Cough is Type B 1. **Mechanism:** ACE inhibitors block the breakdown of **bradykinin**, leading to its accumulation in the lungs. Bradykinin stimulates cough receptors → dry, persistent cough. 2. **Not dose-dependent:** The cough does not reliably increase with higher doses and does not resolve simply by reducing the dose — it requires discontinuation of the drug. 3. **Idiosyncratic/unpredictable:** Only a subset of patients (5–20%) develop cough despite all patients experiencing ACE inhibition. It is not a direct extension of the primary therapeutic action (lowering blood pressure), making it Type B. 4. **Confirmed by switching:** Resolution on switching to losartan (ARB) confirms the bradykinin-mediated mechanism, but the unpredictable occurrence in a subset of patients is the hallmark of Type B. **High-Yield (KD Tripathi / Rang & Dale):** ACE inhibitor cough is classified as **Type B** because it is an idiosyncratic reaction occurring in susceptible individuals, is **not dose-dependent**, and is unrelated to the primary pharmacological action (antihypertensive effect). Type A reactions are direct, dose-dependent extensions of the drug's main pharmacological effect (e.g., excessive hypotension with antihypertensives). ### What is NOT True - **Option A ("It is a Type A reaction")** — This is **NOT true**. ACE inhibitor cough is Type B, not Type A. - **Option B ("It is dose-dependent and predictable")** — This is also **NOT true** of ACE inhibitor cough; however, Option A is the more direct false statement about classification. - **Option C ("Bradykinin accumulation")** — This IS true; bradykinin accumulation is the established mechanism. - **Option D ("It is a Type B reaction independent of dose")** — This IS true. **Clinical Pearl:** The question asks which statement is NOT true. Option A (calling it Type A) is the false statement. Option D correctly identifies it as Type B. **Mnemonic:** **Type A = Augmented** (exaggerated normal effect, dose-dependent); **Type B = Bizarre** (idiosyncratic, dose-independent, occurs in susceptible individuals).