A 19-year-old male collegiate wrestler is brought to the student health center due to prof… — USMLE Step 2 CK

A $19$ -year-old male collegiate wrestler is brought to the student health center due to profound fatigue, dizziness, and muscle cramps over the past two weeks. He has been intensely focused on "making weight" for the upcoming wrestling season. His BMI is $21.5 \text{ kg/m}^2$ . Vital signs show temperature $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), pulse $112$ /min, respirations $12$ /min, and blood pressure $96/60 \text{ mm Hg}$ . Physical examination reveals dry mucous membranes, bilateral parotid gland swelling, and calluses on the dorsum of his right hand. The lungs are clear to auscultation, and the heart has a regular rhythm with no murmurs.

Laboratory studies show:
Sodium: $138 \text{ mEq/L}$
Potassium: $2.6 \text{ mEq/L}$
Chloride: $86 \text{ mEq/L}$
Bicarbonate: $36 \text{ mEq/L}$
Thyroid-stimulating hormone (TSH): $1.2 \text{ }\mu\text{U/mL}$ (normal $0.4$ - $4.0$ )
Free T4: $0.7 \text{ ng/dL}$ (normal $0.9$ - $2.4$ )

An arterial blood gas on room air shows pH $7.52$ and PaCO2 $48 \text{ mm Hg}$ . An ECG reveals sinus tachycardia with flattened T waves and prominent U waves.

Which of the following is the most accurate statement regarding this patient's condition?

The arterial carbon dioxide level reflects appropriate physiological compensation.Answer
B
The arterial blood gas values reveal a superimposed primary respiratory acidosis.
C
Bupropion is the preferred long-term pharmacologic therapy for this patient's disorder.
D
The physician must notify the university athletic department without the patient's consent.
E
The thyroid function tests indicate that primary hypothyroidism is causing the fatigue.

Answer

The arterial carbon dioxide level reflects appropriate physiological compensation.

The correct answer accurately interprets the patient's acid-base status. The patient's self-induced vomiting (suggested by parotid swelling and Russell sign) has caused a hypokalemic, hypochloremic metabolic alkalosis. In metabolic alkalosis, the expected respiratory compensation (hypoventilation) is calculated as: Expected PaCO2 =

40 + [0.7 \times (\text{HCO}_3^- - 24)] \pm 2

. For this patient, Expected PaCO2 =

40 + 0.7(12) = 48.4 \pm 2 \text{ mm Hg}

. The patient's measured PaCO2 of

48 \text{ mm Hg}

falls perfectly within this range, indicating appropriate respiratory compensation without a superimposed respiratory acid-base disorder.

Step-by-Step Solution

Identify the primary psychiatric and medical diagnosis.

The patient has Bulimia Nervosa, evidenced by normal BMI, purging signs (Russell sign, parotid swelling), and volume depletion.

Recognizing the eating disorder contextualizes the electrolyte abnormalities, specifically vomiting-induced hypokalemia and metabolic alkalosis.

Analyze the primary acid-base disturbance.

The arterial blood gas shows alkalemia (pH

7.52

) with a high bicarbonate (

36 \text{ mEq/L}

), confirming a primary metabolic alkalosis.

Vomiting causes loss of gastric acid (HCl) and volume depletion, driving aldosterone-mediated bicarbonate retention and potassium wasting.

Calculate the expected respiratory compensation for the metabolic alkalosis.

Expected PaCO2 =

40 + [0.7 \times (36 - 24)] = 40 + 8.4 = 48.4 \text{ mm Hg} \pm 2

To determine if there is a mixed acid-base disorder, the measured PaCO2 (

48 \text{ mm Hg}

) must be compared to the expected PaCO2 (

46.4 - 50.4 \text{ mm Hg}

). Since it falls within the range, compensation is appropriate.

Key Concept

Metabolic complications of purging behavior and acid-base compensation calculations

Alternative Method

When evaluating complex patients, group the findings by organ system. The calluses and parotid swelling quickly isolate the psychiatric diagnosis (Bulimia Nervosa). The labs isolate the nephrology diagnosis (metabolic alkalosis). Evaluating the answers sequentially against these isolated domains saves time.

Estimated Time:1m 30s

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