A 58-year-old man with hypertension and type 2 diabetes is started on ramipril for blood pressure control and renal protection. The drug acts by inhibiting the enzyme marked **B** in the diagram. After 2 weeks, the patient develops a persistent dry cough and reports throat irritation. Which of the following best explains the mechanism behind this adverse effect?

Explanation

## Why "Inhibition of ACE prevents breakdown of bradykinin, leading to accumulation and airway irritation" is right ACE (angiotensin-converting enzyme) is a dipeptidyl carboxypeptidase with dual substrate specificity. It not only converts angiotensin I to the potent vasoconstrictor angiotensin II, but also inactivates bradykinin—a potent inflammatory mediator and vasodilator. When ACE inhibitors like ramipril block this enzyme, angiotensin II production decreases (therapeutic effect), but simultaneously bradykinin accumulates in the lungs and airways. Elevated bradykinin stimulates vagal C-fiber receptors in the respiratory epithelium, triggering the characteristic dry, persistent cough seen in 10–20% of ACE inhibitor users. This is a well-established class effect documented in Guyton & Hall and KD Tripathi. ## Why each distractor is wrong - **Direct stimulation of AT1 receptors in the respiratory tract causes bronchoconstriction**: ACE inhibitors *block* the formation of angiotensin II, so AT1 receptors are *under*-stimulated, not over-stimulated. This would cause bronchodilation, not cough. The cough is not mediated by AT1 receptor activation. - **Reduced angiotensin II production leads to unopposed vasodilation in pulmonary vessels**: While ACE inhibitors do reduce angiotensin II and cause vasodilation, this mechanism does not explain the dry cough. Pulmonary vasodilation would not trigger airway irritation or the characteristic vagal cough reflex. - **Decreased aldosterone secretion causes electrolyte imbalance affecting airway smooth muscle**: Aldosterone suppression is a downstream consequence of reduced angiotensin II, but electrolyte changes do not cause dry cough. Hyperkalemia (a known side effect of ACE inhibitors) does not produce cough; the cough is specifically due to bradykinin accumulation. **High-Yield:** ACE inhibitors cause dry cough via bradykinin accumulation, not angiotensin II reduction—remember "ACE breaks down both angiotensin I AND bradykinin." [cite: Guyton & Hall Textbook of Medical Physiology, 14e, Ch 19; KD Tripathi Essentials of Medical Pharmacology, 9e, Ch 36]