**Core Concept**
Procainamide is an anti-arrhythmic medication that can affect the heart's electrical activity. In patients with chronic heart disease, it's crucial to consider potential interactions with other medications, especially those affecting cardiac function or electrolyte balance.

**Why the Correct Answer is Right**
Procainamide can slow the heart rate and prolong the QT interval, which may increase the risk of torsades de pointes. Digoxin, a cardiac glycoside, can also prolong the QT interval and increase the risk of arrhythmias. When used together, these medications can synergistically increase the risk of cardiac arrhythmias, making it essential to monitor cardiac function closely in patients receiving both procainamide and digoxin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because procainamide does not significantly interact with hydrochlorothiazide, a diuretic that can lead to hypokalemia, which is already being supplemented by potassium.

**Option B:** This option is incorrect because procainamide does not directly interact with potassium supplementation, which is being taken to manage hypokalemia caused by hydrochlorothiazide.

**Option C:** This option is incorrect because while procainamide can affect cardiac function, it does not directly interact with the patient's rheumatoid arthritis treatment.

**Clinical Pearl / High-Yield Fact**
When prescribing anti-arrhythmic medications like procainamide, always consider potential interactions with other cardiac medications, such as digoxin, and closely monitor cardiac function to prevent adverse effects.

**Correct Answer:** C.