A 32-year-old man is brought to the emergency department due to a 3-day history of progres… — USMLE Step 2 CK

A 32-year-old man is brought to the emergency department due to a 3-day history of progressively worsening, severe right upper quadrant abdominal pain and abdominal swelling. Over the past year, he has experienced significant fatigue and several episodes where his urine appeared abnormally dark, particularly upon waking in the morning. His temperature is $37.8^\circ\text{C}$ ( $100.0^\circ\text{F}$ ), blood pressure is $112/68 \text{ mm Hg}$ , and pulse is $108/\text{min}$ . Physical examination reveals scleral icterus, mildly labored breathing, and a markedly distended abdomen with shifting dullness. There is severe tenderness to palpation in the right upper quadrant. The liver edge is palpable $4 \text{ cm}$ below the right costal margin.

Laboratory results are as follows:

Test	Result	Reference Range
Hemoglobin	$8.5 \text{ g/dL}$	$13.5-17.5 \text{ g/dL}$
Mean corpuscular volume	$92 \text{ fL}$	$80-100 \text{ fL}$
Platelet count	$105,000/\text{mm}^3$	$150,000-400,000/\text{mm}^3$
Leukocyte count	$3,800/\text{mm}^3$	$4,500-11,000/\text{mm}^3$
Reticulocyte count	$8\%$	$0.5-1.5\%$
Total bilirubin	$5.1 \text{ mg/dL}$	$0.1-1.2 \text{ mg/dL}$
Direct bilirubin	$0.6 \text{ mg/dL}$	$0.0-0.3 \text{ mg/dL}$
Lactate dehydrogenase	$1,450 \text{ U/L}$	$100-250 \text{ U/L}$
Haptoglobin	$<10 \text{ mg/dL}$	$41-165 \text{ mg/dL}$

Abdominal duplex ultrasonography shows a prominently enlarged liver with moderate ascites and absent color flow signal within the hepatic veins.

Which of the following therapies directly targets the primary etiology of this patient's current condition?

A
Initiation of a small molecule inhibitor of Janus kinase 2
B
Administration of broad-spectrum intravenous antibiotics
Administration of a monoclonal antibody against complement protein C5Answer
D
Intravenous loop diuretic therapy and fluid restriction
E
Immediate exploratory laparotomy with surgical decompression

Answer

Administration of a monoclonal antibody against complement protein C5

The correct answer is the administration of a monoclonal antibody against complement protein C5 (Eculizumab). This patient's presentation of intravascular hemolysis (anemia, elevated LDH, low haptoglobin, unconjugated hyperbilirubinemia, reticulocytosis), pancytopenia (low platelets, low WBCs), morning hemoglobinuria, and hepatic vein thrombosis (Budd-Chiari syndrome) is highly characteristic of Paroxysmal Nocturnal Hemoglobinuria (PNH). PNH is an acquired clonal hematopoietic stem cell disorder caused by a mutation in the PIGA gene, which leads to a deficiency of glycosylphosphatidylinositol (GPI) anchor proteins (most notably CD55 and CD59). The absence of these regulatory proteins on the cell surface makes red blood cells highly susceptible to complement-mediated destruction. Thrombosis, particularly in atypical sites such as the hepatic, cerebral, or mesenteric veins, is the leading cause of morbidity and mortality in PNH. Eculizumab binds to the terminal complement protein C5, preventing the formation of the membrane attack complex. This effectively halts intravascular hemolysis and significantly reduces the risk of further thrombotic events.

Step-by-Step Solution

Analyze the clinical presentation and laboratory findings to identify the primary mechanism of disease.

The patient has severe right upper quadrant pain, hepatomegaly, and ascites. Labs reveal pancytopenia, marked intravascular hemolysis (high LDH, low haptoglobin, unconjugated hyperbilirubinemia, reticulocytosis), and a history of morning dark urine (hemoglobinuria).

Identifying the constellation of findings is essential to narrow the differential diagnosis for acute hepatomegaly with severe anemia.

Interpret the diagnostic imaging in the context of the clinical picture.

Abdominal duplex ultrasound shows absent color flow in the hepatic veins, confirming the diagnosis of Budd-Chiari syndrome (hepatic vein thrombosis).

Budd-Chiari syndrome is a life-threatening complication that frequently occurs in the setting of underlying hypercoagulable states, guiding the search for a hematologic disorder.

Synthesize the findings of intravascular hemolysis, pancytopenia, and atypical venous thrombosis.

The combination of Coombs-negative intravascular hemolysis, pancytopenia, and intra-abdominal venous thrombosis strongly points to Paroxysmal Nocturnal Hemoglobinuria (PNH).

PNH is a clonal stem cell disorder characterized by a lack of GPI-anchored complement inhibitors (CD55, CD59), leading to unregulated complement-mediated RBC lysis and a highly thrombogenic state.

Determine the targeted pharmacotherapy for the identified condition.

Eculizumab, a monoclonal antibody targeting complement protein C5, is the definitive therapy to halt complement-mediated hemolysis and reduce thrombotic risk in PNH.

Blocking the terminal complement cascade prevents the formation of the membrane attack complex, which is the root pathogenesis in PNH.

Key Concept

Paroxysmal Nocturnal Hemoglobinuria (PNH) classic triad (hemolytic anemia, pancytopenia, and venous thrombosis) and its targeted management with C5 complement inhibitors.

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