A 23-year-old female presents to the emergency department complaining of profound fatigue,… — USMLE Step 3

A $23$ -year-old female presents to the emergency department complaining of profound fatigue, muscle weakness, and severe dizziness whenever she stands up. She works as a professional dancer and admits to being under immense stress before an upcoming audition. On physical examination, her supine blood pressure is $102/64$ mm Hg with a heart rate of $92$ /min. Upon standing, her blood pressure drops to $84/52$ mm Hg and her heart rate increases to $122$ /min. Her mucous membranes are dry, and skin turgor is decreased.

Laboratory evaluation reveals the following:

Parameter	Value	Reference Range
Serum Chemistry
Sodium ( $Na^+$ )	$138$ mEq/L	$136-145$ mEq/L
Potassium ( $K^+$ )	$2.2$ mEq/L	$3.5-5.0$ mEq/L
Chloride ( $Cl^-$ )	$86$ mEq/L	$98-106$ mEq/L
Bicarbonate ( $HCO_3^-$ )	$42$ mEq/L	$22-28$ mEq/L
Blood urea nitrogen (BUN)	$45$ mg/dL	$7-18$ mg/dL
Creatinine	$1.4$ mg/dL	$0.6-1.2$ mg/dL
Spot Urine Chemistry
Sodium ( $Na^+$ )	$68$ mEq/L	variable
Potassium ( $K^+$ )	$55$ mEq/L	variable
Chloride ( $Cl^-$ )	$6$ mEq/L	variable
pH	$7.5$	$4.5-8.0$

Which of the following is the most likely etiology of this patient's clinical and laboratory findings?

Surreptitious self-induced emesisAnswer
B
Covert loop diuretic abuse
C
Defective sodium-chloride cotransport
D
Autonomous aldosterone overproduction
E
Chronic excessive laxative ingestion

Answer

Surreptitious self-induced emesis

This patient presents with a severe hypokalemic metabolic alkalosis, hypovolemia, and acute kidney injury. When evaluating metabolic alkalosis, spot urine chloride is the pivotal test to differentiate between chloride-responsive (urine Cl < 20 mEq/L) and chloride-resistant (urine Cl > 20 mEq/L) etiologies. This patient's critically low urine chloride (6 mEq/L) indicates intense renal chloride conservation due to total body chloride depletion and hypovolemia, characteristic of upper gastrointestinal fluid loss (vomiting). The remarkably high urine sodium (68 mEq/L) in a volume-depleted patient is a classic diagnostic pitfall. During active vomiting, the rapid generation of bicarbonate exceeds the proximal tubule's reabsorptive capacity. As the excess negatively charged bicarbonate is excreted in the urine (evidenced by the alkaline urine pH of 7.5), it obligatorily drags cations, primarily sodium and potassium, with it to maintain tubular electroneutrality. Therefore, in active vomiting, urine sodium is misleadingly elevated despite severe systemic hypovolemia, leaving urine chloride as the only reliable metric to confirm the diagnosis.

Step-by-Step Solution

Identify the primary systemic abnormalities based on vital signs and serum chemistry.

The patient has severe orthostatic hypotension and a BUN/Creatinine ratio of 32:1 indicating profound hypovolemia (prerenal azotemia). Serum labs demonstrate a severe non-anion gap metabolic alkalosis with marked hypokalemia.

Establishing the primary acid-base and volume status is the required first step before interpreting the urinary electrolytes.

Analyze the spot urine chloride to categorize the metabolic alkalosis.

The spot urine chloride is critically low at 6 mEq/L. This categorizes the condition as a chloride-responsive (saline-responsive) metabolic alkalosis.

In metabolic alkalosis, urine chloride is the most accurate metric to distinguish between volume-depleted (saline-responsive, Cl < 20) and volume-expanded/tubular defect (saline-resistant, Cl > 20) etiologies.

Reconcile the paradoxically high spot urine sodium with the clinical evidence of volume depletion.

The urine sodium is inappropriately high (68 mEq/L) for a hypovolemic state. However, the urine pH is alkaline (7.5), indicating active excretion of excess bicarbonate (bicarbonaturia). Because bicarbonate is an anion, it obligatorily drags cations (sodium and potassium) into the urine to maintain electroneutrality.

This confirms that during the active generation phase of vomiting, urine sodium is a highly misleading indicator of volume status, making urine chloride the sole reliable marker.

Synthesize the laboratory data with the clinical vignette.

Severe volume depletion, chloride-responsive metabolic alkalosis, high urine sodium driven by bicarbonaturia, and a very low urine chloride point definitively to active gastric acid loss.

This physiologic profile perfectly matches surreptitious self-induced emesis in a highly stressed patient with an eating disorder.

Key Concept

During the active phase of vomiting, bicarbonaturia obligates the renal excretion of sodium, causing a paradoxically high urine sodium despite severe hypovolemia. In this setting, a low urine chloride (< 10 mEq/L) is the only reliable laboratory indicator of total body volume and chloride depletion.

Estimated Time:2m 30s

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