A 6-year-old boy with sickle cell disease (HbSS) is brought to the emergency department du… — USMLE Step 2 CK

A $6$ -year-old boy with sickle cell disease (HbSS) is brought to the emergency department due to increasing fatigue and generalized weakness over the past $48$ hours. One week ago, he had a low-grade fever accompanied by a bright red "slapped-cheek" facial rash that has since faded. He reports no pain in his chest, abdomen, or extremities. His temperature is $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), blood pressure is $95/60 \text{ mm Hg}$ , pulse is $135/\text{min}$ , and respirations are $22/\text{min}$ . Physical examination reveals marked conjunctival pallor and a grade $2/6$ systolic flow murmur at the left upper sternal border. The abdomen is soft and non-tender, and the spleen is not palpable. Laboratory studies show:

Laboratory Test	Result
Hemoglobin	$3.9 \text{ g/dL}$ (baseline $7.5 \text{ g/dL}$ )
Mean corpuscular volume	$86 \text{ fL}$
Platelets	$310,000/\text{mm}^3$
Leukocytes	$8,500/\text{mm}^3$
Reticulocytes	$0.1\%$ (baseline $8\%-10\%$ )

Which of the following is the most likely underlying mechanism for this patient's acute hematologic presentation?

Transient arrest of erythropoiesis by a viral pathogenAnswer
B
Acute pooling of erythrocytes within the splenic sinusoids
C
Accelerated antibody-mediated destruction of circulating erythrocytes
D
Microvascular occlusion resulting in diffuse bone marrow ischemia
E
Exhaustion of hematopoietic stem cells due to chronic hemolysis

Answer

The correct answer is the transient arrest of erythropoiesis by a viral pathogen, which describes an aplastic crisis triggered by Parvovirus B19.

The correct answer accurately describes the pathophysiology of an aplastic crisis. Parvovirus B19 (the cause of Erythema infectiosum or 'Fifth disease', marked by a 'slapped-cheek' rash) has a strong tropism for erythroid progenitor cells in the bone marrow. In healthy individuals, the resulting transient halt in erythropoiesis goes largely unnoticed. However, in patients with sickle cell disease whose red blood cells have a drastically shortened lifespan (e.g.,

10

20

days), this temporary arrest leads to a precipitous drop in hemoglobin. The hallmark laboratory finding is a profoundly low reticulocyte count in the setting of severe anemia.

Step-by-Step Solution

Analyze the patient's symptoms and vital signs.

6

-year-old boy with sickle cell disease presents with severe, symptomatic anemia (tachycardia, flow murmur, fatigue) without any pain or splenomegaly.

To establish the primary clinical problem, which is an acute decompensation of his chronic baseline anemia.

Evaluate the laboratory findings, paying special attention to the reticulocyte count.

The hemoglobin has dropped severely to

3.9 \text{ g/dL}

, and the reticulocyte count is

0.1\%

, which is inappropriately low for a patient with chronic hemolytic anemia.

To differentiate between hyperproliferative causes of acute anemia (e.g., splenic sequestration, hyperhemolysis) and hypoproliferative causes (e.g., aplastic crisis).

Correlate the hematologic findings with the patient's recent infectious history.

The patient recently had a 'slapped-cheek' rash, which is pathognomonic for Erythema infectiosum caused by Parvovirus B19.

To definitively identify the etiology. Parvovirus B19 specifically infects erythroid progenitor cells, causing a temporary halt in red blood cell production that is life-threatening in patients with shortened RBC lifespans.

Key Concept

An aplastic crisis in sickle cell disease is typically triggered by Parvovirus B19 infection, which specifically targets erythroid progenitor cells and halts red blood cell production, presenting as severe acute anemia with a dangerously low reticulocyte count.

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