A 28-year-old woman comes to the clinic reporting progressive fatigue, mild shortness of b… — USMLE Step 2 CK

A 28-year-old woman comes to the clinic reporting progressive fatigue, mild shortness of breath on exertion, and tingling in her fingertips over the past two weeks. She has a long-standing history of binge-eating disorder characterized by eating large amounts of food with a sense of loss of control, without any compensatory purging behaviors. Three weeks ago, her psychiatrist initiated a new medication that is also indicated for migraine prophylaxis to help manage her binge-eating episodes and promote weight loss. Her body mass index (BMI) is $32.4 \text{ kg/m}^2$ . Her temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), pulse is $88/\text{min}$ , blood pressure is $118/76\text{ mm Hg}$ , and respirations are $18/\text{min}$ .

Laboratory studies reveal the following:

Test	Result
Serum sodium	$141\text{ mEq/L}$
Serum potassium	$3.8\text{ mEq/L}$
Serum chloride	$115\text{ mEq/L}$
Serum bicarbonate	$16\text{ mEq/L}$
Blood urea nitrogen	$14\text{ mg/dL}$
Serum creatinine	$0.9\text{ mg/dL}$

Arterial blood gas analysis on room air shows:

Test	Result
pH	$7.31$
PaCO2	$32\text{ mm Hg}$
PaO2	$95\text{ mm Hg}$

Which of the following best describes the acid-base disturbance in this patient resulting from her new psychiatric medication?

Normal anion gap metabolic acidosis with appropriate respiratory compensationCevap
B
Normal anion gap metabolic acidosis with concurrent respiratory alkalosis
C
Normal anion gap metabolic acidosis with concurrent respiratory acidosis
D
High anion gap metabolic acidosis with appropriate respiratory compensation
E
Metabolic alkalosis with appropriate respiratory compensation

Cevap

Normal anion gap metabolic acidosis with appropriate respiratory compensation

The correct answer identifies a normal anion gap metabolic acidosis with appropriate respiratory compensation. The clinical vignette describes a patient with binge-eating disorder who was recently started on topiramate, an anticonvulsant frequently used off-label for weight reduction in binge-eating disorder and indicated for migraine prophylaxis. Topiramate inhibits carbonic anhydrase in the proximal renal tubule, leading to decreased bicarbonate reabsorption and a subsequent Type 2 renal tubular acidosis. This presents as a normal anion gap metabolic acidosis (hyperchloremic acidosis). The patient's anion gap is calculated as

141 - (115 + 16) = 10 \text{ mEq/L}

, which is within the normal limits of 8-12 mEq/L. To determine if the respiratory compensation is adequate, Winter's formula is applied: Expected PaCO2 =

(1.5 \times 16) + 8 \pm 2 = 32 \pm 2 \text{ mm Hg}

. Because the measured PaCO2 is exactly

32 \text{ mm Hg}

, the patient has an appropriately compensated simple metabolic acidosis without any concurrent respiratory disorders.

Adım Adım Çözüm

Identify the primary psychiatric medication and its associated adverse effects.

The patient with binge-eating disorder is taking topiramate (indicated for weight loss and migraine prophylaxis). A classic side effect of topiramate is paresthesias and a type 2 renal tubular acidosis due to carbonic anhydrase inhibition.

Understanding the pharmacotherapy for eating disorders is required to anticipate the physiologic complications presented in the laboratories.

Determine the primary acid-base disturbance and calculate the serum anion gap.

The pH is 7.31 (acidemia) and serum bicarbonate is 16 mEq/L, indicating a primary metabolic acidosis. The anion gap is 141 - (115 + 16) = 10 mEq/L, which is a normal anion gap.

Calculating the anion gap differentiates the cause of the metabolic acidosis. A normal anion gap confirms the topiramate-induced bicarbonate loss rather than an unmeasured anion accumulation.

Calculate the expected respiratory compensation using Winter's formula.

Expected PaCO2 = (1.5 × HCO3) + 8 ± 2. Substituting the values: (1.5 × 16) + 8 = 24 + 8 = 32 ± 2 mm Hg. The measured PaCO2 is 32 mm Hg.

Comparing the measured PaCO2 to the expected PaCO2 determines if a secondary respiratory acid-base disorder is present. Since they match perfectly, there is appropriate respiratory compensation.

Anahtar Kavram

Topiramate is a pharmacologic option for binge-eating disorder that can cause a normal anion gap metabolic acidosis (Type 2 RTA) due to carbonic anhydrase inhibition. Winter's formula [Expected PaCO2 = (1.5 × HCO3) + 8 ± 2] is used to verify appropriate respiratory compensation in metabolic acidosis.

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