A 58-year-old man with hypertension and chronic kidney disease (eGFR 35 mL/min/1.73 m²) is started on antihypertensive therapy. Which single laboratory finding best distinguishes the use of a beta-blocker from a calcium channel blocker in this patient?

Explanation

## Distinguishing Cardiac and Metabolic Effects ### Cardiac Effects: Beta-Blockers vs Calcium Channel Blockers **Key Point:** Beta-blockers **reduce heart rate** and **slow AV nodal conduction** (prolonged PR interval), whereas calcium channel blockers have variable effects depending on the subclass: - **Non-dihydropyridines** (verapamil, diltiazem): also reduce heart rate and prolong PR interval - **Dihydropyridines** (amlodipine, nifedipine): cause reflex tachycardia, no AV nodal effects **High-Yield:** In a patient on a **non-dihydropyridine CCB**, the cardiac signature (bradycardia + PR prolongation) mimics beta-blockers. However, **dihydropyridine CCBs cause tachycardia**, making them readily distinguishable from beta-blockers. ### Why This Question Assumes Non-Dihydropyridine CCB The stem asks for the "best discriminator" without specifying the CCB subclass. The most clinically relevant distinction is between **beta-blockers and dihydropyridine CCBs**, where: - **Beta-blocker:** bradycardia, PR prolongation - **Dihydropyridine CCB:** tachycardia (reflex), normal PR interval ### Comparison Table: Cardiac and Metabolic Effects | Feature | Beta-Blockers | Non-DHP CCB | DHP CCB | | --- | --- | --- | --- | | **Heart rate** | ↓ (bradycardia) | ↓ (bradycardia) | ↑ (reflex tachycardia) | | **PR interval** | Prolonged | Prolonged | Normal | | **Hyperkalemia risk** | No | No | No | | **Hyperglycemia** | Yes (non-selective) | No | No | | **Serum creatinine** | Minimal change | Minimal change | Minimal change | **Clinical Pearl:** In CKD, **beta-blockers and non-dihydropyridine CCBs are both acceptable**, but dihydropyridine CCBs (e.g., amlodipine) are preferred because they do not cause bradycardia and have neutral metabolic effects. ### Why Serum Potassium Elevation Is NOT the Answer Neither beta-blockers nor CCBs directly elevate potassium in the way ACE inhibitors or ARBs do. Beta-blockers may cause mild hyperkalemia by reducing catecholamine-mediated K^+^ shift into cells, but this is not a primary or reliable discriminator in CKD. ### Why Hyperglycemia Favors Beta-Blockers **Mnemonic:** **BAD** — **B**eta-blockers cause **A**dverse **D**iabetic effects (hyperglycemia, impaired glucose tolerance, especially non-selective agents). CCBs are metabolically neutral. However, **hyperglycemia is an adverse effect to avoid**, not a distinguishing feature used to identify which drug was given. The ECG finding (bradycardia + PR prolongation) is the most **objective and immediately observable** discriminator.