A 31-year-old man is evaluated in the emergency department for a 1-week history of worseni… — USMLE Step 2 CK

A $31$ -year-old man is evaluated in the emergency department for a $1$ -week history of worsening right upper quadrant abdominal pain and abdominal swelling. He also reports profound generalized fatigue and notes that his urine is dark reddish-brown in the morning, which clears as the day progresses. His temperature is $37.3^\circ\text{C}$ ( $99.1^\circ\text{F}$ ), blood pressure is $132/84\text{ mmHg}$ , and pulse is $98/\text{min}$ . Physical examination reveals mild scleral icterus and a tender, distended abdomen with shifting dullness. Hepatomegaly is appreciated, but there is no guarding or rebound tenderness.

Laboratory studies show:

Test	Result
Hemoglobin	$8.4\text{ g/dL}$
Leukocytes	$3,200/\text{mm}^3$
Platelets	$105,000/\text{mm}^3$
Reticulocytes	$8\%$
Total bilirubin	$3.8\text{ mg/dL}$
Indirect bilirubin	$3.1\text{ mg/dL}$
Lactate dehydrogenase	$2,100\text{ U/L}$
Haptoglobin	$<10\text{ mg/dL}$
Aspartate aminotransferase (AST)	$85\text{ U/L}$
Alanine aminotransferase (ALT)	$92\text{ U/L}$

Doppler ultrasonography of the abdomen demonstrates ascites and an absence of blood flow in the hepatic veins.

Which of the following is the most likely underlying mechanism of this patient's condition?

A
Microangiopathic mechanical destruction of erythrocytes
B
Inherited deficiency of a natural circulating anticoagulant
Absence of glycosylphosphatidylinositol-anchored proteins on cell surfacesAnswer
D
IgG autoantibodies directed against red blood cell surface antigens
E
Constitutive activation of a non-receptor tyrosine kinase

Answer

The correct answer is the absence of glycosylphosphatidylinositol-anchored proteins on cell surfaces, which describes the pathophysiology of Paroxysmal Nocturnal Hemoglobinuria (PNH).

The correct answer describes the pathophysiology of Paroxysmal Nocturnal Hemoglobinuria (PNH). PNH is an acquired clonal hematopoietic stem cell disorder characterized by a mutation in the PIGA gene. This mutation leads to an inability to synthesize glycosylphosphatidylinositol (GPI) anchors, which are required to attach various proteins to cell membranes. Crucially, the absence of GPI-anchored proteins includes the complement regulatory proteins CD55 (decay-accelerating factor) and CD59 (MAC-inhibitory protein). Without these inhibitors, red blood cells are highly susceptible to complement-mediated intravascular hemolysis, especially at night when mild respiratory acidosis naturally occurs (leading to the classic 'dark morning urine'). PNH classically presents with a triad of intravascular hemolysis, pancytopenia (due to bone marrow suppression), and a hypercoagulable state. Venous thrombosis in atypical locations, such as the hepatic veins (Budd-Chiari syndrome), portal, or cerebral veins, is a hallmark of the disease and a leading cause of mortality.

Step-by-Step Solution

Analyze the clinical presentation and laboratory findings.

The patient has a classic triad of intravascular hemolysis (high LDH, low haptoglobin, dark morning urine), pancytopenia (low hemoglobin, leukocytes, and platelets), and venous thrombosis (Budd-Chiari syndrome).

To identify the defining syndrome presented in the vignette.

Evaluate the etiology of Budd-Chiari syndrome in the context of the patient's cell counts.

While myeloproliferative neoplasms (like Polycythemia Vera) commonly cause Budd-Chiari syndrome, they present with elevated cell counts. This patient has pancytopenia, pointing toward bone marrow failure syndromes.

To differentiate between the major hematologic causes of hepatic vein thrombosis.

Connect the combination of intravascular hemolysis, pancytopenia, and atypical venous thrombosis to a single diagnosis.

This specific triad is the hallmark of Paroxysmal Nocturnal Hemoglobinuria (PNH).

To arrive at the definitive diagnosis before reviewing the answer choices.

Identify the underlying molecular mechanism of PNH.

PNH is caused by an acquired mutation in the PIGA gene, leading to a deficiency of glycosylphosphatidylinositol (GPI) anchors. This results in the absence of GPI-anchored complement inhibitors (CD55 and CD59) on the cell surface, making erythrocytes susceptible to complement-mediated destruction.

To match the clinical diagnosis with the correct pathophysiological mechanism described in the options.

Key Concept

Paroxysmal nocturnal hemoglobinuria (PNH) presents with the classic triad of intravascular hemolysis, pancytopenia, and venous thrombosis (often in atypical locations such as hepatic or cerebral veins), caused by a deficiency of GPI-anchored complement inhibitors (CD55, CD59) on cell surfaces.

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