Ans. b (Magnesium hydroxide) (Ref. KDT 6th/pg. 635).Mag. hydroxide has low water solubility: its aqueous suspension (milk of magnesia) has low concentration of OH- ions and thus low alkalinity. However, it reacts with HCI promptly and is an efficacious antacid (1 g mEq HCI). Rebound acidity is mild and brief. All Mg salts have a laxative action-by generating osmotically active MgCI, in the stomach and through Mg2+ ion induced cholecystokinin release. Soluble Mg salts are used as osmotic purgatives.Sodium bicarbonateIt reacts rapidly with HCI to produce carbon dioxide and NaCI. Formation of carbon dioxide results in gastric distention and belching. Unreacted alkali is readily absorbed, potentially causing metabolic alkalosis when given in high doses or to patients with renal insufficiency. Sodium chloride absorption may exacerbate fluid retention in patients with heart failure, hypertension, and renal insufficiency.Calcium carbonateIt is less soluble and reacts more slowly than sodium bicarbonate with HCI to form carbon dioxide and CaCI2. Calcium carbonate is used for a number of other indications apart from its antacid properties (bone mineral homeostasis). Excessive doses of either sodium bicarbonate or calcium carbonate with calcium-containing dairy products can lead to hypercalcemia, renal insufficiency, and metabolic alkalosis (milk-alkali syndrome).Magnesium hydroxide or aluminum hydroxideReact slowly with HCI to form magnesium chloride or aluminum chloride and water.Because no gas is generated, belching does not occur.Metabolic alkalosis is also uncommon because of the efficiency of the neutralization reaction.Unabsorbed magnesium salts may cause an osmotic diarrhea and aluminum salts may cause constipation patients with renal insufficiency should not take these agents long-term.