A 21-year-old woman is evaluated in the emergency department after fainting during a balle… — USMLE Step 2 CK

A 21-year-old woman is evaluated in the emergency department after fainting during a ballet rehearsal. Over the past month, she has experienced profound fatigue, recurrent dizziness, and muscle cramps. The patient states she feels immense pressure to maintain a thin physique for her upcoming dance performance. Her mother, who accompanied her, mentions finding several empty packages of bisacodyl in the patient's bathroom. The patient exercises for three hours daily and rigidly restricts her caloric intake, although she admits to occasionally consuming large quantities of food before feeling intensely guilty.

Vital signs show a temperature of $36.4^\circ\text{C}$ ( $97.5^\circ\text{F}$ ), blood pressure of 92/58 mm Hg, and pulse of 114/min. Her current body mass index (BMI) is $17.0 \text{ kg/m}^2$ .

Physical examination reveals poor skin turgor and hyperactive bowel sounds.

Laboratory results are as follows:
Serum sodium: 139 mEq/L
Serum potassium: 2.4 mEq/L
Serum chloride: 115 mEq/L
Serum bicarbonate: 14 mEq/L
Blood urea nitrogen: 26 mg/dL
Serum creatinine: 1.1 mg/dL

Arterial blood gas analysis on room air:
pH: 7.27
$PaCO_2$ : 30 mm Hg
$PaO_2$ : 96 mm Hg

Which of the following represents the most accurate psychiatric diagnosis and the correct interpretation of this patient's acid-base status?

Anorexia nervosa, binge-eating/purging subtype; normal anion gap metabolic acidosis with appropriate respiratory compensationAnswer
B
Anorexia nervosa, binge-eating/purging subtype; normal anion gap metabolic acidosis with a superimposed primary respiratory alkalosis
C
Bulimia nervosa; normal anion gap metabolic acidosis with appropriate respiratory compensation
D
Anorexia nervosa, restricting subtype; high anion gap metabolic acidosis with appropriate respiratory compensation
E
Bulimia nervosa; high anion gap metabolic acidosis with a superimposed primary respiratory alkalosis

Answer

Anorexia nervosa, binge-eating/purging subtype; normal anion gap metabolic acidosis with appropriate respiratory compensation

The correct answer accurately identifies both the psychiatric condition and the acid-base disturbance. The patient's significantly low body weight (BMI of 17.0 kg/m^2) combined with an intense fear of weight gain confirms anorexia nervosa, while her objective binges and laxative abuse specify the binge-eating/purging subtype. Laxative abuse causes significant gastrointestinal loss of potassium and bicarbonate, resulting in a normal anion gap metabolic acidosis. The anion gap is calculated as 139 - (115 + 14) = 10 mEq/L, which is within the normal range of 8-12 mEq/L. To evaluate for appropriate respiratory compensation, Winter's formula is applied: expected PaCO2 = 1.5 * (14) + 8 +/- 2 = 29 +/- 2 mm Hg. The patient's measured PaCO2 of 30 mm Hg falls exactly within this expected compensatory range.

Step-by-Step Solution

Identify the primary psychiatric diagnosis based on physical measurements and behavior.

The patient's BMI is 17.0 kg/m^2 (below the 18.5 kg/m^2 threshold), establishing anorexia nervosa. Her episodes of eating large quantities of food and using bisacodyl confirm the binge-eating/purging subtype.

Differentiating anorexia nervosa from bulimia nervosa relies heavily on the BMI threshold; diagnosing the subtype relies on the presence of binge/purge behaviors versus pure restriction.

Determine the primary acid-base disorder from the ABG and basic metabolic panel.

The pH of 7.27 indicates acidemia, and the low serum bicarbonate of 14 mEq/L confirms a primary metabolic acidosis, commonly caused by laxative-induced bicarbonate loss in the stool.

Identifying the primary disturbance is the first step in acid-base interpretation.

Calculate the anion gap to classify the metabolic acidosis.

Anion Gap = Na+ - (Cl- + HCO3-) = 139 - (115 + 14) = 139 - 129 = 10 mEq/L. This falls in the normal range (8-12 mEq/L).

Calculating the anion gap distinguishes between hyperchloremic acidosis (diarrhea/laxatives) and high anion gap acidosis (starvation ketoacidosis).

Apply Winter's formula to determine if respiratory compensation is appropriate.

Expected PaCO2 = 1.5 * (14) + 8 = 29 (+/- 2). The expected range is 27-31 mm Hg. Since the patient's measured PaCO2 is 30 mm Hg, the respiratory compensation is appropriate.

Winter's formula determines if there is a mixed acid-base disorder. If the measured PaCO2 falls within the expected range, no superimposed respiratory disorder is present.

Key Concept

Patients with anorexia nervosa, binge-eating/purging subtype due to laxative abuse present with a BMI < 18.5 kg/m^2 and typically develop a normal anion gap (hyperchloremic) metabolic acidosis with appropriate respiratory compensation.

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