A 25-year-old female fashion model is evaluated in the emergency department for severe mus… — USMLE Step 2 CK

A 25-year-old female fashion model is evaluated in the emergency department for severe muscle cramps, dizziness, and palpitations. She says her symptoms began this morning and have progressively worsened. The patient is under immense pressure from her agency to meet strict body measurement requirements for an upcoming international campaign. Vital signs show a temperature of $36.8^\circ$ C ( $98.2^\circ$ F), heart rate of $115$ /min, respiratory rate of $20$ /min, and blood pressure of $102/62$ mm Hg. BMI is $19.5$ kg/m $^2$ . Physical examination reveals dry mucous membranes and hyperactive bowel sounds. There is no parotid gland enlargement, and the dorsal surfaces of her hands are unremarkable.

Laboratory results are as follows:

Test	Result
Serum sodium	$140$ mEq/L
Serum potassium	$2.4$ mEq/L
Serum chloride	$116$ mEq/L
Serum bicarbonate	$14$ mEq/L
Blood urea nitrogen	$32$ mg/dL
Serum creatinine	$1.2$ mg/dL

Arterial blood gas (room air):

Parameter	Result
pH	$7.28$
pCO $_2$	$29$ mm Hg
pO $_2$	$95$ mm Hg

Which of the following is the most likely cause of this patient's current presentation?

Frequent self-administration of bisacodylCevap
B
Self-induced vomiting with concomitant respiratory failure
C
Severe caloric restriction leading to starvation ketosis
D
Surreptitious ingestion of excessive levothyroxine
E
Co-ingestion of fluoxetine and phenelzine

Cevap

The correct answer is frequent self-administration of bisacodyl, which is a stimulant laxative.

The correct answer describes surreptitious laxative abuse. This patient has a primary metabolic acidosis (pH

7.28

, low

\text{HCO}_3^-

). Her anion gap is calculated as

140 - (116 + 14) = 10

, which indicates a normal anion gap metabolic acidosis (NAGMA). The

\text{pCO}_2

29

mm Hg reflects appropriate respiratory compensation according to Winter's formula (Expected

\text{pCO}_2 = 1.5 \times 14 + 8 = 29

). In a young woman with immense professional pressure to maintain a low body weight, a hypokalemic NAGMA combined with signs of volume depletion (BUN:Cr ratio

> 20

) and hyperactive bowel sounds strongly indicates surreptitious laxative abuse. Laxatives stimulate rapid colonic transit, leading to profound gastrointestinal loss of both potassium and bicarbonate.

Adım Adım Çözüm

Analyze the primary acid-base disturbance using the arterial blood gas.

The patient has a low pH (

7.28

) and low bicarbonate (

14

mEq/L), indicating a primary metabolic acidosis.

Establishing the primary disorder is the mandatory first step in evaluating any electrolyte/acid-base derangement.

Calculate the serum anion gap to narrow the differential diagnosis.

Anion Gap =

\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (116 + 14) = 10

mEq/L. This falls within the normal range (

8-12

mEq/L).

Differentiating between High Anion Gap (HAGMA) and Normal Anion Gap (NAGMA) completely changes the underlying physiological etiology.

Determine if the respiratory compensation is appropriate using Winter's formula.

Expected

\text{pCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2 = 1.5 \times 14 + 8 \pm 2 = 29 \pm 2

mm Hg. The actual

\text{pCO}_2

29

mm Hg, indicating perfect respiratory compensation.

This rules out any concomitant primary respiratory acidosis or alkalosis (e.g., respiratory failure or primary hyperventilation).

Synthesize the lab findings (hypokalemic NAGMA, high BUN/Cr ratio > 20) with the clinical vignette (young woman under pressure to be thin, hyperactive bowel sounds).

The combined picture of gastrointestinal bicarbonate and potassium loss, resulting in a volume-depleted NAGMA, strongly points to surreptitious laxative abuse.

Eating disorder patients often use different purging methods. Differentiating them relies heavily on specific lab patterns: vomiting causes alkalosis, laxatives cause acidosis.

Anahtar Kavram

Surreptitious laxative abuse in eating disorders presents clinically with volume depletion, severe hypokalemia, and a normal anion gap metabolic acidosis (NAGMA) due to massive gastrointestinal bicarbonate loss.

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