A 20-year-old collegiate gymnast is evaluated at the university clinic after reporting sev… — USMLE Step 2 CK

A 20-year-old collegiate gymnast is evaluated at the university clinic after reporting several weeks of profound muscle weakness, generalized fatigue, and intermittent muscle cramps. Her vital signs are temperature $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), heart rate 94/min, blood pressure 105/65 mm Hg, and respiratory rate 18/min. BMI is 19.5 kg/m $^2$ . Physical examination reveals dry mucous membranes and diminished deep tendon reflexes bilaterally. Her dental enamel is completely intact, and there are no calloused lesions on the dorsal aspects of her hands. The parotid glands are not enlarged.

Laboratory studies are obtained and show the following:

Arterial Blood Gas (Room Air)	Result	Reference Range
pH	7.29	7.35-7.45
$pCO_2$	29 mm Hg	35-45 mm Hg
$pO_2$	95 mm Hg	80-100 mm Hg

Serum Chemistry	Result	Reference Range
Sodium	138 mEq/L	136-145 mEq/L
Potassium	2.7 mEq/L	3.5-5.0 mEq/L
Chloride	114 mEq/L	95-105 mEq/L
Bicarbonate	13 mEq/L	22-28 mEq/L
Blood urea nitrogen	22 mg/dL	7-18 mg/dL
Creatinine	1.0 mg/dL	0.6-1.2 mg/dL
Glucose	85 mg/dL	70-110 mg/dL

Which of the following compensatory behaviors is most likely responsible for this patient's clinical and laboratory findings?

Frequent use of stimulant laxativesAnswer
B
Self-induced vomiting after meals
C
Surreptitious use of loop diuretics
D
Prolonged periods of absolute fasting
E
Intentional omission of exogenous insulin

Answer

The correct answer is frequent use of stimulant laxatives.

The correct answer is frequent use of stimulant laxatives. This patient presents with a normal body mass index, hypokalemia (

2.7 \text{ mEq/L}

), and a normal anion gap (hyperchloremic) metabolic acidosis. The anion gap is calculated as

138 - (114 + 13) = 11 \text{ mEq/L}

, which is normal. In the context of a young female collegiate athlete, this presentation is highly suggestive of an eating disorder with purging behaviors. Laxatives cause significant gastrointestinal losses of bicarbonate (

HCO_3^-

) and potassium (

K^+

), leading directly to a hypokalemic, normal anion gap metabolic acidosis. The absence of dental erosions, parotid swelling, and Russell sign (calluses on hands) further points away from vomiting as the method of purging.

Step-by-Step Solution

Analyze the arterial blood gas and serum chemistry to determine the primary acid-base disorder.

The patient has a low pH (7.29) and a low bicarbonate (

13 \text{ mEq/L}

), indicating a metabolic acidosis. The

pCO_2

of 29 mm Hg reflects an appropriate respiratory compensation.

Identifying the primary disturbance is the first step in narrowing the differential diagnosis for electrolyte abnormalities.

Calculate the serum anion gap (AG).

AG = Na^+ - (Cl^- + HCO_3^-) = 138 - (114 + 13) = 11 \text{ mEq/L}

. This is within the normal range (8-12 mEq/L), indicating a normal anion gap (hyperchloremic) metabolic acidosis.

The anion gap distinguishes between different etiologies of metabolic acidosis (e.g., normal AG vs. elevated AG).

Correlate the normal anion gap metabolic acidosis and profound hypokalemia with the clinical presentation.

A normal BMI athlete presenting with weakness, hypokalemia, and normal AG metabolic acidosis without signs of vomiting (no dental erosions, no parotid swelling) strongly suggests laxative abuse.

Different purging methods cause distinct electrolyte profiles; distinguishing laxative abuse (acidosis) from vomiting (alkalosis) is a critical diagnostic skill.

Key Concept

Bulimia nervosa (purging type via laxative abuse) presents with hypokalemia and a normal anion gap metabolic acidosis due to gastrointestinal loss of potassium and bicarbonate.

Estimated Time:1m 30s

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