USMLE PRACTICE QUESTION #98
TOPIC: Infectious disease.
Today’s question will test your understanding of a commonly tested infectious disease.
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A 29-year-old female presents to the clinic with myalgias, recurring fevers, and headaches two weeks after returning from a trip to Africa. Before her travels, she was prescribed prophylactic chloroquine. Her fevers have been occurring randomly, with the highest recorded temperature at 39.3°C (102.8°F). Physical examination shows jaundice and splenomegaly. Her hemoglobin concentration is measured at 8 g/dL. A peripheral blood smear shows several red blood cells with dark aggregations of chromatin. Which of the following is the most likely cause of her symptoms?
A) Influenza virus
B) Plasmodium falciparum
C) Haemophilus influenza
D) Streptococcus pneumonia
E) Dengue fever
F) SARS-CoV-2
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
The MENTAL MODEL used to answer this question comes from our detailed test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:
Step 1. Read the last line to get to the heart of the question: “Which of the following is the most likely cause of her symptoms?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 1st order. 1st: Make a diagnosis based on the given scenario.
Step 3: Read the vignette carefully and ask yourself: “Based on the presentation, what’s the most likely cause?”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
This vignette describes a 29-year-old female who presents with myalgias, recurring fevers, headaches, and jaundice two weeks after returning from a trip to Africa. She was prescribed prophylactic chloroquine before her travels, but her symptoms suggest that the prophylaxis may not have been effective. The patient also has splenomegaly and anemia (hemoglobin of 8 g/dL), common findings in malaria.
ANSWER CHOICES:
CHOICE A: Influenza virus
Explanation: Influenza virus causes respiratory symptoms such as fever, cough, sore throat, and myalgias. However, it is not associated with jaundice, splenomegaly, or anemia. This patient’s travel history to Africa, along with her systemic findings (fever, jaundice, splenomegaly), makes malaria a much more likely diagnosis than influenza.
CHOICE B: Plasmodium falciparum
Explanation: Plasmodium falciparum is the most severe form of malaria and is common in sub-Saharan Africa. It can cause severe hemolysis, leading to anemia, jaundice, and splenomegaly. The hallmark finding on blood smear for P. falciparum is the presence of multiple infected red blood cells with ring forms or "headphone" appearances and dark chromatin aggregations.
CHOICE C: Haemophilus influenzae
Explanation: Haemophilus influenzae is a bacterial pathogen that can cause respiratory infections such as pneumonia or meningitis. It does not typically cause systemic symptoms like jaundice or splenomegaly.
CHOICE D: Streptococcus pneumoniae
Explanation: Streptococcus pneumoniae is a common cause of bacterial pneumonia and meningitis but does not typically cause jaundice, splenomegaly, or anemia. While S. pneumoniae can cause fever and fatigue, it does not explain this patient’s travel history or the findings of jaundice and splenomegaly. Additionally, there are no respiratory symptoms or signs of meningitis in this case.
CHOICE E: Dengue fever
Explanation: Dengue fever is a viral illness transmitted by mosquitoes that causes high fever, headache, retro-orbital pain, myalgias ("breakbone fever"), and sometimes hemorrhagic manifestations like petechiae or bleeding. While dengue can cause fever and myalgias in travelers returning from endemic areas (such as Africa), it does not typically cause jaundice or splenomegaly. Additionally, dengue fever does not explain the findings on peripheral blood smear (abnormal RBCs with chromatin aggregations).
CHOICE F: SARS-CoV-2
Explanation: This patient’s presentation—fever after travel to Africa with jaundice and splenomegaly—is more consistent with malaria than COVID-19. SARS-CoV-2 does not typically cause anemia or abnormal RBC findings on blood smear.
FINAL VERDICT…
The most likely cause of this patient’s symptoms is infection with Plasmodium falciparum, the most severe form of malaria. Key points supporting this diagnosis include:
Travel History to Africa: Malaria is endemic in many parts of Africa.
Prophylactic Chloroquine Use: While chloroquine can be used for malaria prophylaxis in some regions, P. falciparum has developed widespread resistance to chloroquine in many parts of Africa.
Fever Pattern: The patient has recurring fevers at random times—this irregular pattern is typical for P. falciparum malaria.
Jaundice and Splenomegaly: These are classic signs of hemolysis caused by malaria.
Anemia (Hemoglobin 8 g/dL): Malaria causes hemolysis, leading to anemia.
Peripheral Blood Smear Findings: The description of red blood cells with dark chromatin aggregations suggests parasitized RBCs typical for P. falciparum. The presence of multiple infected RBCs per field is another hallmark finding.
Plasmodium falciparum malaria explains all aspects of this patient’s presentation—fever after travel to an endemic area, jaundice due to hemolysis, splenomegaly from sequestration of infected RBCs in the spleen, anemia from hemolysis, and characteristic findings on peripheral blood smear.
CORRECT ANSWER: B) Plasmodium falciparum
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That’s it for question 98!
See ya tomorrow 👋