A 68-year-old female presents to the outpatient clinic reporting progressive fatigue and s… — USMLE Step 2 CK

A 68-year-old female presents to the outpatient clinic reporting progressive fatigue and shortness of breath with minimal exertion over the past three months. Her past medical history includes chronic obstructive pulmonary disease and primary hypothyroidism. Her current medications include inhaled tiotropium and oral levothyroxine. Vital signs are within normal limits. Pulse oximetry is $96\%$ on room air. Physical examination reveals marked conjunctival pallor. Lung fields are clear to auscultation bilaterally, and there is no jugular venous distention or peripheral edema.

Laboratory studies reveal the following:
Hemoglobin: $7.1\text{ g/dL}$
Mean corpuscular volume: $115\text{ fL}$
White blood cell count: $4,500/\text{mm}^3$
Platelet count: $160,000/\text{mm}^3$
Thyroid-stimulating hormone: $2.4\text{ }\mu\text{U/mL}$ (normal: $0.4-4.0\text{ }\mu\text{U/mL}$ )

Which of the following is the most appropriate next step in management?

Measure serum vitamin B12 and folate levelsAnswer
B
Increase the daily dosage of levothyroxine
C
Prescribe an oral macrolide antibiotic and a corticosteroid burst
D
Perform an echocardiogram and initiate a loop diuretic
E
Obtain an arterial blood gas to assess acid-base status

Answer

Measure serum vitamin B12 and folate levels

The correct answer accurately identifies the next step for a patient presenting with an elevated mean corpuscular volume (MCV > 100 fL). The most common causes of macrocytic anemia are vitamin B12 and folate deficiencies, and these levels should be obtained as the first step in the diagnostic workup.

Step-by-Step Solution

Identify the primary clinical abnormality based on laboratory findings.

The patient has symptomatic severe anemia (hemoglobin

7.1\text{ g/dL}

) with macrocytosis (mean corpuscular volume

115\text{ fL}

Recognizing the predominant laboratory abnormality directs the diagnostic algorithm.

Evaluate the clinical presentation against the patient's comorbidities.

The normal thyroid-stimulating hormone rules out uncontrolled hypothyroidism. The clear lungs and normal oxygen saturation rule out an acute cardiopulmonary exacerbation.

This ensures that symptoms of fatigue and dyspnea are correctly attributed to the anemia rather than chronic diseases.

Select the appropriate initial test for the identified anemia.

Vitamin B12 and folate levels are indicated.

These are the most common causes of macrocytic anemia and must be evaluated first to guide definitive therapy.

Key Concept

The initial diagnostic step for macrocytic anemia (elevated mean corpuscular volume) includes checking serum vitamin B12 and folate levels.

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