Metabolic abnormality seen in gastric outlet obstruction is
## **Core Concept**
Gastric outlet obstruction leads to a specific set of metabolic abnormalities due to the accumulation of gastric contents, which are rich in hydrochloric acid (HCl), and the inability to properly absorb nutrients. The primary issue revolves around the loss of hydrogen ions (H+) and chloride (Cl-) ions, along with decreased absorption of other essential nutrients.
## **Why the Correct Answer is Right**
The correct answer, **Hypochloremic, hypokalemic metabolic alkalosis**, accurately describes the metabolic abnormality seen in gastric outlet obstruction. This condition arises because:
- The obstruction prevents gastric emptying, leading to persistent vomiting or gastric aspiration.
- Vomiting results in the loss of HCl (hydrochloric acid), which is rich in hydrogen (H+) and chloride (Cl-) ions.
- The loss of H+ ions leads to a decrease in hydrogen ions in the blood, causing a rise in blood pH, which characterizes metabolic alkalosis.
- To compensate for the alkalosis and maintain electroneutrality, the kidneys retain more bicarbonate (HCO3-) and excrete more hydrogen ions, potassium ions (K+), and hydrogen ions are exchanged for potassium in the renal tubules, leading to hypokalemia (low potassium levels).
- Chloride levels decrease due to direct loss through vomiting.
## **Why Each Wrong Option is Incorrect**
- **Option A (Hyperchloremic metabolic acidosis):** This is incorrect because gastric outlet obstruction typically leads to loss of HCl, not an increase, and results in metabolic alkalosis, not acidosis.
- **Option B (Normochloremic metabolic acidosis):** This option is incorrect because the chloride levels are usually decreased (hypochloremia) due to HCl loss, and metabolic acidosis is not the typical acid-base disturbance seen.
- **Option D (Hyperchloremic, hypokalemic metabolic acidosis):** This is incorrect for similar reasons as Option A; the acid-base disturbance is alkalosis, not acidosis, and chloride levels are decreased.
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the metabolic alkalosis seen in gastric outlet obstruction often requires correction of the underlying cause (relieving the obstruction) and replacement of lost fluids, electrolytes (especially potassium and chloride), and sometimes acid (as HCl) to correct the severe hypochloremia and hypokalemia.
## **Correct Answer: C. Hypochloremic, hypokalemic metabolic alkalosis**