A 27-year-old male actor comes to the clinic for a routine evaluation prior to starting fi… — USMLE Step 2 CK

A 27-year-old male actor comes to the clinic for a routine evaluation prior to starting filming for a new movie. Over the past 5 months, he has purposefully lost $14 \text{ kg}$ ( $30.8 \text{ lb}$ ) by restricting his caloric intake to fewer than $500 \text{ kcal/day}$ and engaging in strenuous cardiovascular exercise for 3 hours daily. He states, "I have to look absolutely emaciated for this role, but I feel so weak and dizzy when I stand up." He strongly denies binge eating, self-induced vomiting, or the use of laxatives or diuretics. His body mass index is $16.2 \text{ kg/m}^2$ . Vital signs show a blood pressure of 88/58 mm Hg and a heart rate of 48/min. Physical examination reveals temporal wasting, dry skin, and fine, downy hair on his upper back.

Laboratory studies show:

Test	Result	Reference Range
Sodium	$139 \text{ mEq/L}$	$136-145 \text{ mEq/L}$
Potassium	$3.4 \text{ mEq/L}$	$3.5-5.0 \text{ mEq/L}$
Chloride	$104 \text{ mEq/L}$	$95-105 \text{ mEq/L}$
Bicarbonate	$15 \text{ mEq/L}$	$22-28 \text{ mEq/L}$
Blood urea nitrogen	$28 \text{ mg/dL}$	$7-18 \text{ mg/dL}$
Creatinine	$1.0 \text{ mg/dL}$	$0.6-1.2 \text{ mg/dL}$
Glucose	$62 \text{ mg/dL}$	$70-99 \text{ mg/dL}$

Arterial blood gas (room air):

Parameter	Result	Reference Range
pH	$7.30$	$7.35-7.45$
pCO2	$30 \text{ mm Hg}$	$33-45 \text{ mm Hg}$
pO2	$96 \text{ mm Hg}$	$75-105 \text{ mm Hg}$

Which of the following best describes this patient's primary acid-base disturbance and its underlying etiology?

A
High anion gap metabolic acidosis with concurrent primary respiratory alkalosis secondary to starvation ketosis
B
Normal anion gap metabolic acidosis with appropriate respiratory compensation secondary to surreptitious laxative abuse
High anion gap metabolic acidosis with appropriate respiratory compensation secondary to starvation ketosisCevap
D
Metabolic alkalosis with appropriate respiratory compensation secondary to self-induced vomiting
E
Normal anion gap metabolic acidosis with concurrent primary respiratory acidosis secondary to surreptitious laxative abuse

Cevap

The patient has a high anion gap metabolic acidosis with appropriate respiratory compensation secondary to starvation ketosis.

The correct answer identifies a high anion gap metabolic acidosis with appropriate respiratory compensation due to starvation ketosis. The patient is acidemic (pH = 7.30) with a primary metabolic acidosis (bicarbonate = 15 mEq/L). The anion gap is elevated at 20 mEq/L (

139 - (104 + 15) = 20

), confirming a high anion gap metabolic acidosis. In the setting of severe caloric restriction, this is caused by starvation ketosis (accumulation of beta-hydroxybutyrate and acetoacetate). To check for a mixed disorder, Winter's formula is applied:

\text{Expected pCO}_2 = 1.5(HCO_3) + 8 \pm 2

. For this patient,

\text{Expected pCO}_2 = 1.5(15) + 8 \pm 2 = 30.5 \pm 2

, meaning the expected pCO2 range is 28.5 to 32.5 mm Hg. The patient's actual pCO2 of 30 mm Hg falls squarely within this range, indicating appropriate physiological respiratory compensation without any concurrent primary respiratory disorder.

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Analyze the patient's clinical presentation.

The patient has anorexia nervosa, restricting subtype, with severe caloric restriction, hypoglycemia, and signs of profound malnutrition.

Identifying the severe fasting state points toward starvation ketosis as the likely metabolic derangement.

Evaluate the primary acid-base disturbance.

The pH of 7.30 indicates acidemia, and the low bicarbonate of 15 mEq/L confirms a primary metabolic acidosis.

The initial step in acid-base analysis is determining the primary disorder using pH and bicarbonate levels.

Calculate the serum anion gap.

AG = Na - (Cl + HCO_3) = 139 - (104 + 15) = 20 \text{ mEq/L}

Categorizing the metabolic acidosis as high or normal anion gap narrows the differential diagnosis. A gap of 20 is elevated.

Calculate the expected respiratory compensation using Winter's formula.

\text{Expected pCO}_2 = 1.5(HCO_3) + 8 \pm 2 = 1.5(15) + 8 \pm 2 = 30.5 \pm 2 \text{ mm Hg}

To determine if a secondary respiratory acid-base disorder is present, the expected pCO2 must be compared to the actual pCO2.

Synthesize the findings.

The actual pCO2 of 30 mm Hg is within the expected range of 28.5 to 32.5 mm Hg, confirming appropriate respiratory compensation without a concurrent primary respiratory disorder.

Integrating the laboratory calculations with the clinical context provides the final diagnosis.

Anahtar Kavram

Evaluation of acid-base disturbances in eating disorders requires calculating the anion gap and Winter's formula to differentiate between starvation ketosis, laxative abuse, and vomiting.

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