A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 17-year-old girl is brought to her pediatrician by her mother for a wellness checkup. The patient states she is doing well in school and has no concerns. She has a past medical history of anxiety and is currently taking clonazepam as needed. Her family history is remarkable for hypertension in her mother and father and renal disease in her grandparents and aunt. Her temperature is 98.6°F (37.0°C), blood pressure is 97/68 mmHg, pulse is 90/min, respirations are 9/min, and oxygen saturation is 99% on room air. The patient's BMI is 23 kg/m^2. Cardiac, pulmonary, and neurological exams are within normal limits. Laboratory values are ordered as seen below.
Hemoglobin: 10 g/dL
Hematocrit: 29%
Leukocyte count: 6,500/mm^3 with normal differential
Platelet count: 190,000/mm^3
Serum:
Na+: 137 mEq/L
Cl-: 97 mEq/L
K+: 3.5 mEq/L
HCO3-: 29 mEq/L
BUN: 20 mg/dL
Glucose: 67 mg/dL
Creatinine: 1.1 mg/dL
Ca2+: 10.2 mg/dL
Urine:
pH: 4.5
Color: yellow
Glucose: none
Chloride: 4 mEq/L
Sodium: 11 mEq/L
Which of the following is the most likely diagnosis?
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A
Anorexia nervosaIncorrect. Anorexia nervosa requires significantly low body weight (BMI typically <17.5); this patient's BMI is 23, in the normal range.
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B
Bulimia nervosaCorrect. Normal BMI with hypokalemic metabolic alkalosis and very low urine chloride is the classic picture of bulimia nervosa with self-induced vomiting (HCl loss conserves urinary chloride).
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C
Diuretic abuseIncorrect. Diuretic abuse would cause HIGH urine chloride because the diuretic blocks chloride reabsorption.
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D
Gitelman syndromeIncorrect. Gitelman syndrome causes hypokalemic alkalosis but typically with hypomagnesemia and HIGH urine chloride from constitutive distal-tubule sodium-chloride loss, not the low urine chloride seen here.
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E
Liddle syndromeIncorrect. Liddle syndrome causes hypokalemic alkalosis and hypertension from constitutive ENaC activity, but this patient is normotensive and her urine chloride is very low, both inconsistent with Liddle.
↑ Tap an answer to reveal the reasoning
Answer: B. A teenage girl with anemia, mild metabolic alkalosis (HCO3 29), borderline-low potassium (3.5), normal BMI, and a urine chloride of only 4 mEq/L is the textbook presentation of bulimia nervosa with self-induced vomiting. The key clue is the urine chloride: in vomiting, gastric HCl loss produces metabolic alkalosis with LOW urine chloride (<20 mEq/L) — the kidney conserves chloride to preserve volume. In diuretic abuse, urine chloride is HIGH because the diuretic blocks reabsorption.
Bulimia is characterized by recurrent binges followed by compensatory behaviors (vomiting, laxatives, diuretics, excessive exercise), with a typically NORMAL or near-normal BMI distinguishing it from anorexia nervosa. The metabolic alkalosis here suggests vomiting. Other features that may be present include parotid swelling, dental erosion (perimolysis), Russell sign (knuckle calluses from inducing vomiting), and electrolyte derangements.
Distinguishers: anorexia nervosa shows low BMI (this girl's BMI is 23, normal); diuretic abuse shows HIGH urine chloride (not 4); Gitelman syndrome causes hypokalemia, alkalosis, hypomagnesemia, and HYPOcalciuria, but also presents with HIGH urine chloride from continuous distal tubule sodium-chloride loss. The pearl: hypokalemic metabolic alkalosis with LOW urine chloride = vomiting (bulimia); HIGH urine chloride = diuretics or Bartter/Gitelman.
**Why each option:**
**A.** Anorexia nervosa requires significantly low body weight (BMI typically <17.5); this patient's BMI is 23, in the normal range.
**B.** Correct. Normal BMI with hypokalemic metabolic alkalosis and very low urine chloride is the classic picture of bulimia nervosa with self-induced vomiting (HCl loss conserves urinary chloride).
**C.** Diuretic abuse would cause HIGH urine chloride because the diuretic blocks chloride reabsorption.
**D.** Gitelman syndrome causes hypokalemic alkalosis but typically with hypomagnesemia and HIGH urine chloride from constitutive distal-tubule sodium-chloride loss, not the low urine chloride seen here.