**Core Concept**
Gastric outlet obstruction and duodenal ulcers are associated with loss of gastric acid, leading to metabolic alkalosis. The loss of hydrogen ions in the stomach secretions results in a decrease in chloride ions, causing hypochloremic metabolic alkalosis.

**Why the Correct Answer is Right**
The most appropriate therapy for this patient is to replace the lost hydrogen and chloride ions. The administration of a combination of potassium chloride (KCl) or potassium citrate and hydrochloric acid (HCl) or its equivalent (e.g., ammonium chloride or citrate) is essential. The addition of potassium helps to correct the hypokalemia, while the chloride helps to correct the hypochloremic alkalosis. This combination is often referred to as "Push-Pull" therapy, where the chloride is "pushed" into the stomach and the potassium is "pulled" into the cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** Administering only potassium supplements would not address the chloride deficiency and may worsen the metabolic alkalosis.
* **Option B:** Administering only hydrochloric acid (HCl) would not address the potassium deficiency and may worsen the hypokalemia.
* **Option D:** Administering only antacids or H2-receptor antagonists may help to reduce acid secretion but would not address the chloride and potassium deficiencies.

**Clinical Pearl / High-Yield Fact**
In patients with gastric outlet obstruction and duodenal ulcers, it is essential to remember the "Push-Pull" therapy, which involves administering a combination of potassium chloride and hydrochloric acid or its equivalent to correct both the hypochloremic and hypokalemic metabolic alkalosis.

**Correct Answer:** C. A combination of potassium chloride (KCl) and hydrochloric acid (HCl) or its equivalent.