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This patient with an increased anion gap metabolic acidosis was most likely suffering from diabetic ketoacidosis (DKA).  Patients classically have a fruity odor to the breath and often present with mental status changes, dehydration, abdominal pain, and tachypnea.  Laboratory findings include hyperglycemia, ketosis, mild hyponatremia, normal or elevated serum potassium (despite a total body deficit), and increased plasma osmolality.

Insulin and hydration are the primary treatments for DKA.  Insulin allows the cells to use glucose as an energy source, thereby decreasing lipolysis and production of ketone bodies.  Because ketones are the principal acid produced in excess in patients with DKA, decreased production of ketone bodies will result in increased serum bicarbonate.  Insulin also causes an intracellular shift of potassium, resulting in decreased serum potassium levels (patients typically require potassium repletion due to osmotic urinary loss).  In addition to insulin-induced changes, rehydration with normal saline will help normalize serum sodium concentration (by providing isotonic sodium chloride) and decrease serum osmolality (by lowering serum glucose levels).

(Choice B)  Loop diuretics could cause a decrease in potassium concentration as well as an increase in the serum concentration of bicarbonate.  However, they also increase (not decrease) serum osmolality due to increased free water excretion (loop diuretics decrease the medullary concentration gradient, limiting the maximum tonicity of the urine).

(Choice C)  Metabolic acidosis may develop in hypoaldosteronism (type 4 renal tubular acidosis), which is treated with exogenous mineralocorticoids.  However, the combination of an increased anion gap and impaired mental status is not characteristic for hypoaldosteronism.  Treatment with mineralocorticoids causes sodium and water retention with a mild increase (not decrease) in serum osmolality.  Mineralocorticoids also decrease serum potassium and increase serum bicarbonate due to urinary K⁺ and H⁺ loss.

(Choice D)  Opioid antagonists are useful in treating opioid overdoses, which typically cause respiratory acidosis (not anion gap metabolic acidosis) due to hypoventilation.

(Choice E)  Thyroxine supplementation is useful in treating severe hypothyroidism, which may present with hyponatremia, extracellular volume expansion, and hypoglycemia.

Educational objective:
The treatment of choice for diabetic ketoacidosis is intravenous normal saline and insulin.  These therapies increase serum bicarbonate and sodium levels, lower serum glucose and potassium levels, and decrease overall serum osmolarity.