**Core Concept:** In patients with heart failure, particularly those on diuretic therapy, one of the most common side effects is renal tubular acidosis. This occurs due to the inhibition of hydrogen ion secretion by loop diuretics, leading to a decrease in plasma bicarbonate levels.

**Why the Correct Answer is Right:** Here, we are dealing with renal tubular acidosis, which is a consequence of loop diuretic therapy. The correct answer, A) HCO3-, is the plasma bicarbonate level that is most likely decreased in this patient. The loop diuretic is inhibiting hydrogen ion secretion in the nephron, leading to a decrease in the reabsorption of bicarbonate ions. Consequently, the patient experiences symptoms like muscle weakness and elevated plasma HCO3-.

**Why Each Wrong Option is Incorrect:**

B) PCO2 is not the correct answer because the patient's PCO2 is elevated (48 mm Hg), indicating respiratory alkalosis, which is a compensatory response to acidosis but not the primary issue here.

C) pH is not the correct answer as the patient's pH is normal (7.49), suggesting that the acidosis is primarily renal, not respiratory.

D) H+ is not the correct answer because the patient has elevated PCO2 and normal pH, indicating a compensatory respiratory alkalosis, not a primary decrease in plasma hydrogen ions.

**Clinical Pearl:**

In patients on loop diuretic therapy, when they develop symptoms like muscle weakness, it is essential to consider renal tubular acidosis as a potential side effect. Monitoring plasma bicarbonate levels, as in this case, can help identify such side effects and guide therapeutic adjustments.