#72 - THE WEEKLY USMLE TIMES
Highlights for the week of April 22, 2024
Welcome to this week’s issue of The USMLE Times… A special welcome to the 98 new members who have joined our community this week.
Here’s what’s on tap for issue #72 of the USMLE Times:
Question of the Week
Video lesson of the week
Tweet of the week
Links to this week’s training
Question deep-dive & breakdown
Let’s dive in!
A 55-year-old male presents to the emergency department with severe abdominal pain, fever, and diarrhea for the past two days. His medical history includes hypertension and type 2 diabetes mellitus. On physical examination, the patient appears dehydrated, and his abdomen is tender to palpation. Laboratory studies reveal a white blood cell count of 18,000 cells/µL with a left shift. Blood and stool cultures are obtained, and a gram-negative rod-shaped bacterium is identified. Which of the following is the most likely mechanism by which this bacterium causes infection?
A. Production of exotoxins that disrupt host cell membranes
B. Inhibition of protein synthesis in host cells via an A-B toxin
C. Induction of apoptosis in host cells through a Type III secretion system
D. Invasion and replication within host cells, evading immune response
E. Production of endotoxins that stimulate an overwhelming host immune response
F. Adherence to host cells using specialized pili, resulting in colonization
G. Formation of a biofilm that protects the bacteria from host defenses and antibiotics
The answer & question breakdown is at the bottom of the post.
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ANSWER + QUESTION BREAKDOWN
Given the clinical presentation of severe abdominal pain, fever, and diarrhea, along with identifying a gram-negative rod-shaped bacterium in blood and stool cultures, we’re on the lookout for a mechanism that best reflects the most likely causative organism.
ANSWER CHOICES:
ANSWER CHOICE A: Production of exotoxins that disrupt host cell membranes
Exotoxins can directly damage host cells by disrupting membranes. This is common in bacteria like Vibrio cholerae or Clostridium perfringens, which could match some symptoms but would not typically cause a systemic infection observable in blood cultures.
ANSWER CHOICE B: Inhibition of protein synthesis in host cells via an A-B toxin
Bacteria such as Shigella or certain strains of E. coli produce A-B toxins that affect protein synthesis, leading to cell death and typical diarrhea symptoms. However, systemic infection symptoms are less common with this mechanism alone.
ANSWER CHOICE C: Induction of apoptosis in host cells through a Type III secretion system
Certain bacteria like Salmonella or Yersinia use this system to induce apoptosis in host cells, aligning well with the severe abdominal symptoms and systemic signs observed.
ANSWER CHOICE D: Invasion and replication within host cells, evading immune response
This mechanism is typical for bacteria like Salmonella, which invade and replicate within intestinal and macrophage cells, potentially leading to the observed systemic signs, including the high white blood cell count.
ANSWER CHOICE E: Production of endotoxins that stimulate an overwhelming host immune response
Gram-negative bacteria, such as Escherichia coli or Salmonella, contain lipopolysaccharide (LPS) in their outer membrane. LPS, also known as endotoxin, can trigger a robust immune response in the host, leading to inflammation, fever, and sepsis if left untreated.
ANSWER CHOICE F: Adherence to host cells using specialized pili, resulting in colonization
While adherence is crucial for colonization, it does not fully explain the systemic infection and severe symptoms observed on its own.
ANSWER CHOICE G: Formation of a biofilm that protects the bacteria from host defenses and antibiotics
Biofilm formation is a defense mechanism against antibiotics and the immune system but would not directly cause the acute symptoms observed in this scenario.
THE VERDICT: The patient's clinical presentation and identification of a gram-negative rod-shaped bacterium suggest an infection caused by a gram-negative enteric pathogen. A common mechanism of disease in gram-negative bacteria is the production of endotoxins, specifically lipopolysaccharide (LPS), which is a component of the outer membrane of gram-negative bacteria. When released during bacterial growth or death, LPS triggers an overwhelming host immune response, leading to fever, inflammation, and potential septic shock.
FINAL ANSWER: E. Production of endotoxins that stimulate an overwhelming host immune response
That’s it for issue #72 of The USMLE Times!
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