A 62-year-old woman is evaluated in the emergency department for progressive fatigue, exer… — USMLE Step 3

A 62-year-old woman is evaluated in the emergency department for progressive fatigue, exertional dyspnea, and bilateral tingling in her feet over the past three months. She has a history of hypothyroidism treated with levothyroxine and a remote history of Roux-en-Y gastric bypass surgery.

Vital signs are temperature $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), heart rate $105\text{/min}$ , blood pressure $110/70\text{ mmHg}$ , and respiratory rate $16\text{/min}$ . Physical examination reveals conjunctival pallor and mild scleral icterus. Neurologic examination demonstrates decreased vibratory and position sensation in the bilateral lower extremities.

Laboratory results are as follows:

Laboratory Test	Result	Reference Range
Hemoglobin	$6.8\text{ g/dL}$	$12.0-15.5\text{ g/dL}$
Mean corpuscular volume	$122\text{ fL}$	$80-100\text{ fL}$
Platelet count	$95,000/\mu\text{L}$	$150,000-400,000/\mu\text{L}$
Leukocyte count	$4,200/\mu\text{L}$	$4,500-11,000/\mu\text{L}$
Reticulocyte count	$0.4\%$	$0.5-1.5\%$
Lactate dehydrogenase	$2,800\text{ U/L}$	$140-280\text{ U/L}$
Haptoglobin	$8\text{ mg/dL}$	$40-200\text{ mg/dL}$
Total bilirubin	$3.1\text{ mg/dL}$	$0.1-1.2\text{ mg/dL}$
Indirect bilirubin	$2.7\text{ mg/dL}$	$0.1-1.0\text{ mg/dL}$
Prothrombin time (PT)	$12\text{ sec}$	$11-15\text{ sec}$
Partial thromboplastin time (PTT)	$28\text{ sec}$	$25-40\text{ sec}$

Based on the interpretation of these laboratory results, which of the following is the most likely underlying mechanism for this patient's presentation?

Ineffective erythropoiesis resulting in intramedullary precursor destructionAnswer
B
Microangiopathic hemolytic anemia with peripheral platelet consumption
C
Systemic activation of the coagulation cascade with secondary factor depletion
D
IgG-mediated autoantibody binding to erythrocyte surface antigens
E
Immune-mediated destruction of multipotent hematopoietic stem cells

Answer

Ineffective erythropoiesis resulting in intramedullary precursor destruction

The correct answer accurately describes the pathophysiology of severe vitamin B12 deficiency, a condition to which this patient is highly susceptible due to her history of gastric bypass surgery. Vitamin B12 is necessary for nuclear maturation and DNA synthesis. When deficient, hematopoiesis is profoundly impaired, leading to massive apoptosis of megaloblastic erythrocyte precursors within the bone marrow. This intramedullary destruction releases massive amounts of lactate dehydrogenase and unconjugated bilirubin while dropping haptoglobin, a pattern that mimics peripheral hemolytic anemia (often termed pseudo-thrombotic microangiopathy). The critical differentiating laboratory feature is the inappropriately low reticulocyte count; a healthy marrow responding to peripheral destruction would exhibit marked reticulocytosis. Furthermore, the extreme macrocytosis and characteristic neurologic findings (subacute combined degeneration) secure the diagnosis.

Step-by-Step Solution

Interpret the basic hematologic and biochemical laboratory data.

The patient has severe macrocytic anemia, mild thrombocytopenia, elevated LDH, elevated indirect bilirubin, and low haptoglobin.

Identifying the combination of profound LDH elevation, indirect hyperbilirubinemia, and haptoglobin consumption establishes the presence of significant cell breakdown (hemolysis).

Analyze the reticulocyte count in the context of the identified hemolysis.

The reticulocyte count is inappropriately low (

0.4\%

In peripheral hemolysis (e.g., microangiopathic or autoimmune), the functional bone marrow should respond by increasing reticulocyte production. A low reticulocyte count indicates a failure of production or destruction of precursors before they exit the bone marrow.

Correlate the laboratory findings with the clinical history and symptoms.

The combination of severe macrocytosis (

122\text{ fL}

), intramedullary hemolysis, gastric bypass history, and neurologic deficits points to severe Vitamin B12 deficiency.

Vitamin B12 is essential for DNA synthesis. Its absence causes ineffective erythropoiesis where developing megaloblasts undergo apoptosis in the marrow, releasing LDH and bilirubin while producing a low peripheral reticulocyte count. This phenomenon is known as pseudo-thrombotic microangiopathy.

Key Concept

Severe Vitamin B12 deficiency can present with laboratory signs of massive hemolysis (elevated LDH, low haptoglobin, indirect hyperbilirubinemia) mimicking thrombotic microangiopathy; however, the inappropriately low reticulocyte count and extreme macrocytosis correctly identify the process as intramedullary precursor destruction (ineffective erythropoiesis).

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