A 70-year-old woman presents to the emergency department because of anorexia, nausea, and diarrhea. She has had these symptoms for the past 3 days. She recently completed a course of antibiotics for bronchitis. Her medical problems include osteoporosis, type 2 diabetes mellitus, hypertension, glaucoma, and osteoarthritis. Her current medications include metformin, fosinopril, chlorthalidone, ibandronate, acetazolamide, and acetaminophen. Her temperature is 37.4 C (99.4 F), blood pressure is 120/70 mm Hg, pulse is 84/min, and respirations are 16/min. On examination, she appears uncomfortable. Her abdomen is mildly tender in both of the lower quadrants. Test of the stool for blood is positive.
The patient's laboratory studies reveal:

Which of the following is responsible for this patient's acid-base disorder?

Question

A 70-year-old woman presents to the emergency department because of anorexia, nausea, and diarrhea.  She has had these symptoms for the past 3 days.  She recently completed a course of antibiotics for bronchitis.  Her medical problems include osteoporosis, type 2 diabetes mellitus, hypertension, glaucoma, and osteoarthritis.  Her current medications include metformin, fosinopril, chlorthalidone, ibandronate, acetazolamide, and acetaminophen.  Her temperature is 37.4 C (99.4 F), blood pressure is 120/70 mm Hg, pulse is 84/min, and respirations are 16/min.  On examination, she appears uncomfortable.  Her abdomen is mildly tender in both of the lower quadrants.  Test of the stool for blood is positive.
The patient's laboratory studies reveal:

Which of the following is responsible for this patient's acid-base disorder?

Quizplus · Accepted Answer

11ec193a_96a6_bd7b_853d_d56d64d12e10_MD0002_00 This patient has a normal anion gap metabolic acidosis, which occurs due to either renal or gastrointestinal losses of bicarbonate. The kidney responds to metabolic acidosis by increasing renal ammonium excretion, which can be estimated by determining the urinary anion gap (= urinary sodium + urinary potassium − urinary chloride). A negative urinary anion gap indicates a significant amount of an unmeasured positively charged cation (eg, ammonium) in the urine. This patient's urinary anion gap of -36 (= 28 + 20 − 84) suggests a normal renal response to acidosis and a nonrenal cause for the acidosis, in this case diarrhea. (Choice A) Acetazolamide inhibits carbonic anhydrase and can be associated with a normal anion gap metabolic acidosis. The negative urinary anion gap in this patient suggests a gastrointestinal source rather than an effect of acetazolamide. (Choice B) ACE inhibitors can cause acute kidney injury and sometimes hyperkalemia with a mild renal acidification defect that resembles type 4 renal tubular acidosis. The negative urinary anion gap in this patient indicates intact renal acidification. (Choice D) In selected individuals with impaired renal function, metformin can be associated with the development of lactic acidosis, which usually causes an increased anion gap metabolic acidosis. (Choice E) Type 4 renal tubular acidosis results from impaired action of aldosterone at the renal tubular level, and is usually associated with hyperkalemia, often seen in the context of diabetes mellitus. The negative urinary anion gap in this patient indicates an extrarenal source of bicarbonate loss. Educational objective: In the presence of a normal anion gap metabolic acidosis, a negative urinary anion gap suggests a gastrointestinal cause of the acidosis. __________ References: The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis (http://www.ncbi.nlm.nih.gov/pubmed/3344005)

A 70-Year-Old Woman Presents to the Emergency Department Because of Anorexia