Okay, let's tackle this question. The patient has congestive heart failure and was started on an inotropic drug four months ago. Now he's experiencing yellow scotomas and xanthopsia, along with an ECG. The question is about the mechanism of the drug.

First, I need to recall the common inotropic drugs used in heart failure. The main ones are digoxin, dobutamine, milrinone, etc. But digoxin is known for causing visual disturbances like xanthopsia (seeing yellow) and scotomas. Also, digoxin toxicity can lead to ECG changes like prolonged PR interval or various arrhythmias.

The symptoms mentioned here are classic for digoxin toxicity. So the drug in question is likely digoxin. The mechanism of action of digoxin is inhibiting the Na+/K+ ATPase pump. This increases intracellular sodium, which reduces the activity of the Na+/Ca2+ exchanger, leading to increased intracellular calcium and thus increased myocardial contractility.

Now, looking at the options (even though they aren't listed), if the correct answer is about inhibiting Na+/K+ ATPase, that's the one. The other options would be for different drugs. For example, beta-agonists like dobutamine work through beta-1 receptors, while phosphodiesterase inhibitors like milrinone act by increasing cAMP levels.

So the core concept here is the mechanism of digoxin, which is Na+/K+ ATPase inhibition. The clinical pearl is that xanthopsia and yellow scotomas are signs of digoxin toxicity, so this is a high-yield fact for exams. The ECG findings might show other signs like arrhythmias, which also point towards digoxin toxicity.

**Core Concept**
This question tests knowledge of **digoxin toxicity**, a classic adverse effect of long-term inotropic therapy. Digoxin's mechanism involves **inhibition of the Na⁺/K⁺-ATPase pump**, leading to increased intracellular Ca²⁺ and enhanced myocardial contractility, but toxicity manifests as visual disturbances and ECG changes.

**Why the Correct Answer is Right**
The patient's symptoms (yellow scotomas, xanthopsia) and ECG findings are hallmark signs of **digoxin toxicity**. Digoxin inhibits the **Na⁺/K⁺-ATPase pump**, causing intracellular Na⁺ accumulation. This reduces the activity of the **Na⁺/Ca²⁺ exchanger**, leading to elevated cytosolic Ca²⁺ and increased cardiac contractility. However, excess Ca²⁺ also triggers arrhythmias and visual toxicity.

**Why Each Wrong Option is Incorrect**
**Option A:** Beta-adrenergic agonism (e.g., dobutamine) enhances contractility via β₁-receptors but does not cause xanthopsia.
**Option B:** Phosphodiesterase inhibition (e.g., milrinone) increases cAMP, improving contractility without visual side effects.
**Option C:** Calcium channel blockade (e.g., verapamil) reduces contractility, contradicting inotropic therapy.

**Clinical Pearl / High-Yield Fact**
**Xanthopsia and yellow scotomas** are red flags for **digoxin toxicity**. Remember: "Dig

✓ Correct Answer: C. Inhibition of Na-K ATPase pump