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USMLE® Step 1 Question of the Day: Signs of End Organ Damage

Osmosis Team
Published on Feb 10, 2021. Updated on Feb 16, 2021.

Each week, Osmosis shares a USMLE® Step 1-style practice question to test your knowledge of medical topics and help you prepare for the boards. Today's case involves a 64-year-old woman who's experiencing headaches, vision disturbances, and reduced urine output. Can you figure out what the diagnosis could be?

A 64-year-old woman presents to the emergency department because of decreased urine output and dark urine for the last 2 weeks. She also reports fatigue and a worsening headache associated with visual disturbance for the last 2 days. Her past medical history is significant for hypertension, but she states that she has not taken her medication for the past year. Her temperature is 37 °C (98.6 °F), pulse is 86/min, respirations are 20/min, and blood pressure is 222/146 mmHg. Fundoscopic examination shows papilledema and retinal hemorrhages. Urine dipstick is positive for blood. Which of the following laboratory findings is also most likely to be present in this patient?

A. Platelet count of 250,000/μL  

B. Reticulocyte count of 6%

C. Normal levels of haptoglobin

D. Direct bilirubin level of 3 mg/dL

E. Hematocrit of 40%

Scroll down to find the answer!

The correct answer to today's USMLE® Step 1 Question is...

B. Reticulocyte count of 6%

Before we get to the Main Explanation, let's look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

The incorrect answers to today's USMLE® Step 1 Question are...

A. Platelet count of 250,000/μL  

Incorrect: This patient with hypertensive emergency likely has thrombotic microangiopathy. She would be expected to have a decreased platelet count of less than 150,000/μL due to platelet consumption.

C. Normal levels of haptoglobin

Incorrect: This patient would be expected to have low haptoglobin levels of less than 50 mg/dL, due to intravascular hemolytic anemia. As hemolysis occurs, hemoglobin is released into the blood and binds to haptoglobin. Both proteins are removed from the blood together and haptoglobin levels decrease.

D. Direct bilirubin level of 3 mg/dL

Incorrect: This patient would be expected to have normal direct bilirubin levels of less than 0.3 mg/dL. Indirect bilirubin levels would be elevated in this patient due to bleeding and hemolytic anemia. 

E. Hematocrit of 40%

Incorrect: This patient with dark urine and urine dipstick positive for blood would be expected to have a low hematocrit of less than 37% due to hemolytic anemia. Normal hematocrit levels in women are 37–47% and in  42–50% in men.

Main Explanation 

This patient with a blood pressure of 222/146 mmHg associated with signs of end organ damage (severe headache, visual disturbance, as well as papilledema and retinal hemorrhages seen on fundoscopy) is most likely experiencing a hypertensive emergency.  Hypertensive emergency is defined as the combination of severely elevated blood pressure (systolic ≥180 mmHg and diastolic ≥120 mmHg) and signs of end-organ damage, such as stroke, myocardial infarction, aortic dissection, retinal hemorrhages and exudates, acute kidney injury, and microangiopathic hemolytic anemia

This patient’s decreased urine output and darker than normal urine indicate a resultant microangiopathic hemolytic anemia (MAHA). MAHA is a type of normocytic hemolytic anemia, defined as having a mean corpuscular volume (MCV) of 80-100 fL. In hypertensive emergency, fibrinoid necrosis of blood vessels leads to endothelial damage, which causes vessel narrowing and the formation of platelet-rich microthrombi (causing thrombocytopenia) within small vessels. This leads to RBC damage (hemolysis) as the cells pass through these obstructed blood vessels, causing the MAHA. The damaged RBCs can be seen as sheared erythrocyte fragments, or schistocytes, on peripheral blood smear. The resultant hemolytic anemia then presents with darkened urine, which is caused by hemoglobinuria and hemosiderinuria. This usually occurs a few days later when the renal tubular cells, which absorbed the hemoglobin, slough off into the urine. Hemolytic anemia also presents with elevated reticulocyte count, due to a compensatory increase in erythrocyte production.


Major Takeaway 

Hypertensive emergency is defined as the combination of severely elevated blood pressure (systolic ≥180 mmHg and diastolic ≥120 mmHg) and signs of end-organ damage, such as microangiopathic hemolytic anemia (MAHA). MAHA is a type of normocytic hemolytic anemia characterized by the presence of schistocytes on peripheral blood smear. Common laboratory findings include normal MCV, decreased hemoglobin, decreased haptoglobin, and increased reticulocyte count. 

References

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