The parents of a 6-month-old boy of Mediterranean descent bring him to the clinic for a ro… — USMLE Step 2 CK

The parents of a 6-month-old boy of Mediterranean descent bring him to the clinic for a routine evaluation. They report that he has been increasingly fussy, easily fatigued, and feeding poorly over the past month. The parents also express two additional concerns: they are worried because he cannot sit up independently, and they request he receive his measles, mumps, and rubella (MMR) vaccine today to finish his immunizations early. He was born at full term, and his growth has consistently tracked along the 50th percentile. His temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), heart rate is $155/\text{min}$ , and respiratory rate is $36/\text{min}$ . On physical examination, the infant appears notably pale. A grade 2/6 systolic ejection murmur is auscultated at the left lower sternal border. The liver edge is palpated $3\text{ cm}$ below the right costal margin, and the spleen is palpable $4\text{ cm}$ below the left costal margin. Neurologic examination reveals normal symmetric tone; he sits with truncal support and successfully rolls from front to back.

Laboratory testing reveals:
Hemoglobin: $6.2\text{ g/dL}$
Mean corpuscular volume (MCV): $62\text{ fL}$
Reticulocyte count: $7\%$
Thyroid-stimulating hormone (TSH): $2.5\text{ \mu U/mL}$ (normal: $0.5-5.0\text{ \mu U/mL}$ )
Free T4: $1.4\text{ ng/dL}$ (normal: $0.9-2.3\text{ ng/dL}$ )

A peripheral blood smear shows microcytic, hypochromic red blood cells with numerous target cells.

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

Quantitative hemoglobin analysisAnswer
B
Referral for early developmental intervention
C
Administration of the live-attenuated MMR vaccine
D
Empiric initiation of broad-spectrum intravenous antibiotics
E
Initiation of levothyroxine therapy

Answer

Quantitative hemoglobin analysis

The most appropriate next step in the management of this patient is quantitative hemoglobin analysis. This infant's presentation of severe microcytic anemia, target cells, an elevated reticulocyte count, and hepatosplenomegaly at 6 months of age is classic for beta-thalassemia major. The onset of symptoms coincides precisely with the physiologic decline in fetal hemoglobin (HbF) around 4 to 6 months of life. Quantitative hemoglobin analysis (via electrophoresis or chromatography) is the definitive test to confirm the diagnosis by demonstrating an absence or severe decrease of adult hemoglobin (HbA) alongside compensatory elevations in HbF and HbA2.

Step-by-Step Solution

Identify the primary pathological process based on the clinical presentation and laboratory data.

The 6-month-old infant presents with severe microcytic anemia, an elevated reticulocyte count, hepatosplenomegaly, and target cells on the peripheral smear.

Recognizing the combination of microcytic anemia with extramedullary hematopoiesis (hepatosplenomegaly) and target cells is essential for narrowing the differential diagnosis to hemoglobinopathies.

Synthesize the demographic context and symptom timeline with the physical findings.

The patient is of Mediterranean descent, and symptoms appeared at 5 to 6 months of age, which perfectly aligns with the physiologic decline in fetal hemoglobin (HbF) and the unmasking of a beta-globin chain synthesis defect.

This timeline and demographic profile strongly point to beta-thalassemia major as the underlying diagnosis.

Determine the diagnostic gold standard for the suspected condition.

Quantitative hemoglobin analysis (via electrophoresis or high-performance liquid chromatography) is required to evaluate the relative percentages of HbA, HbA2, and HbF.

Identifying abnormal hemoglobin fractions definitively diagnoses beta-thalassemia and guides long-term management, including chronic transfusion therapy.

Evaluate and confidently dismiss distracting clinical findings.

The infant has normal milestones for a 6-month-old, normal thyroid function, and tachycardia/murmur secondary to high-output cardiac states from anemia. Administering the MMR vaccine early is contraindicated.

A physician must prioritize the acute, life-threatening hematologic abnormality over parental concerns regarding normal milestones or inappropriate early vaccine requests.

Key Concept

Diagnosis and management of beta-thalassemia major, characterized by severe microcytic anemia, target cells, and extramedullary hematopoiesis presenting after 4 to 6 months of age as fetal hemoglobin declines.

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