A previously healthy 55-year-old man undergoes elective right hemicolectomy for a stage I (T2N0M0) cancer of the cecum. His postoperative ileus is somewhat prolonged, and on the fifth postoperative day his nasogastric tube is still in place. Physical examination reveals diminished skin turgor, dry mucous membranes, and ohostatic hypotension. Peinent laboratory values are as follows: Aerial blood gases: pH 7.56, PCO2 50 mm Hg, PO2 85 mm Hg. Serum electrolytes (mEq/L): Na + 132, K+ 3.1, Cl – 80; HCO3 – 42. Urine electrolytes (mEq/L): Na + 2, K+ 5, Cl – 6. What is the patient’s acid-base abnormality?

**Core Concept**
The patient's acid-base status is being assessed in the context of a postoperative ileus, where gastrointestinal motility is impaired, leading to the accumulation of acidic gastric contents and potential metabolic disturbances.

**Why the Correct Answer is Right**
The patient's laboratory values reveal a primary increase in bicarbonate (HCO3-) levels (42 mEq/L), indicating a metabolic alkalosis. The high HCO3- levels are associated with a compensatory respiratory alkalosis, as evidenced by the increased PCO2 (50 mm Hg) and a pH of 7.56. The low potassium (K+) levels (3.1 mEq/L) suggest potassium loss, likely due to vomiting or gastric aspiration of acidic contents, which can lead to hypokalemia. The urine electrolyte analysis shows a high potassium excretion (5 mEq/L), supporting the diagnosis of a mineralocorticoid-induced potassium-losing nephropathy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option would be incorrect if the patient's pH was below 7.35, indicating acidosis, which is not the case here.

**Option B:** This option would be incorrect if the patient's PCO2 was low, indicating a respiratory acidosis, which is not the case here.

**Option C:** This option would be incorrect if the patient's bicarbonate levels were decreased, indicating a metabolic acidosis, which is not the case here.

**Clinical Pearl / High-Yield Fact**
In postoperative ileus, metabolic alkalosis can occur due to vomiting or gastric aspiration of acidic contents, leading to potassium loss and hypokalemia. This is a classic example of a mineralocorticoid-induced potassium-losing nephropathy.

**Correct Answer: C. Metabolic alkalosis.**

✓ Correct Answer: D. Metabolic alkalosis with respiratory compensation