A 24-year-old woman comes to the clinic reporting progressive fatigue and generalized weak… — USMLE Step 2 CK

A $24$ -year-old woman comes to the clinic reporting progressive fatigue and generalized weakness that has worsened over the past three months. She notes that her menstrual cycles are regular but feature heavy bleeding, frequently lasting up to $8$ days with the passage of large clots. She takes no daily medications and has no chronic medical conditions. Vital signs show a blood pressure of $112/68$ mm Hg and a heart rate of $102$ /min. Physical examination is notable for pale conjunctivae and a regular tachycardia without murmurs.

Laboratory results are as follows:

Test	Result	Reference Range
Hemoglobin	$8.4$ g/dL	$12.0$ - $15.5$ g/dL
Mean corpuscular volume	$72$ fL	$80$ - $100$ fL
Leukocyte count	$6,200$ /mm $^3$	$4,500$ - $11,000$ /mm $^3$
Platelet count	$420,000$ /mm $^3$	$150,000$ - $400,000$ /mm $^3$

Which of the following is the most appropriate next step in the evaluation of this patient's laboratory findings?

Measurement of serum ferritinAnswer
B
Hemoglobin electrophoresis
C
Measurement of thyroid-stimulating hormone
D
Echocardiography
E
Endometrial biopsy

Answer

Measurement of serum ferritin

The correct next step is measurement of serum ferritin. This patient presents with symptomatic microcytic anemia, highly suggestive of iron deficiency anemia secondary to chronic blood loss from heavy menstrual bleeding. Serum ferritin is the most sensitive and specific initial test for iron deficiency, as it accurately reflects total body iron stores. A low ferritin level confirms the diagnosis.

Step-by-Step Solution

Analyze the patient's symptoms and vital signs.

The patient has progressive fatigue, weakness, and tachycardia, which are classic signs of symptomatic anemia. Her history of heavy menstrual bleeding provides a clear etiology for blood loss.

Establishing the clinical syndrome points toward a hematologic origin for her symptoms rather than a primary cardiac or endocrine issue.

Interpret the laboratory findings.

The patient has a low hemoglobin (

8.4

g/dL) and a low mean corpuscular volume (

72

fL), confirming a microcytic anemia. A reactive thrombocytosis (platelets

420,000

/mm

^3

) is also a common associated finding.

Classifying the anemia by cell size (MCV) narrows the differential diagnosis to microcytic causes, most commonly iron deficiency or thalassemia.

Determine the most appropriate next diagnostic step.

Given the high prevalence of iron deficiency in menstruating women and the classic laboratory picture, measuring serum ferritin is the most accurate initial test to confirm iron stores are depleted.

Confirming the diagnosis of iron deficiency anemia is required before initiating iron supplementation or pursuing further workup for other causes of microcytosis.

Key Concept

Diagnosis of microcytic anemia and initial evaluation of suspected iron deficiency

Estimated Time:1m 0s

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