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Difficulty: HardInfectious Disease Mechanisms

A 52-year-old man who works in a leather processing facility is brought to the intensive care unit with acute respiratory distress and severe lethargy. He reports a 3-day history of a mild, flu-like illness characterized by myalgias and a nonproductive cough. Over the past 12 hours, his condition rapidly deteriorated. His temperature is 39.4°C (102.9°F), blood pressure is 78/40 mm Hg, pulse is 135/min, and respirations are 28/min. Pulse oximetry shows 86% on room air. Physical examination reveals profuse diaphoresis, cyanosis, and diminished breath sounds bilaterally. A chest radiograph demonstrates a significantly widened mediastinum and bilateral pleural effusions. Blood cultures rapidly grow large, boxcar-shaped, Gram-positive, non-motile bacilli in long chains. The responsible pathogen secretes a tripartite exotoxin. Which of the following best describes the precise intracellular mechanism of the exotoxin component primarily responsible for this patient's profound tissue edema and phagocyte dysfunction?

  1. A
    Functions as a zinc-dependent metalloprotease that inactivates mitogen-activated protein kinase kinases
  2. Mimics host adenylate cyclase and requires host intracellular calmodulin for its catalytic activityAnswer
  3. C
    Catalyzes the ADP-ribosylation of an inhibitory G-protein alpha subunit to cause unregulated cyclic AMP production
  4. D
    Binds to host cell surface receptors and oligomerizes to form a transmembrane endosomal pore
  5. E
    Enzymatically degrades host cell membrane phospholipids, resulting in rapid cellular lysis and myonecrosis

Answer

The correct answer is the option stating that the toxin component mimics host adenylate cyclase and requires host intracellular calmodulin for its catalytic activity.
The clinical presentation of a leather worker with severe respiratory distress, shock, a widened mediastinum on imaging, and blood cultures growing boxcar-shaped Gram-positive bacilli in chains is pathognomonic for inhalational anthrax, caused by Bacillus anthracis. B. anthracis produces a tripartite exotoxin consisting of Protective Antigen (PA), Edema Factor (EF), and Lethal Factor (LF). The component responsible for profound edema is EF. Edema Factor functions by mimicking host adenylate cyclase; upon entering the host cell, it requires binding to host calmodulin to become active, after which it rapidly catalyzes the conversion of ATP to cyclic AMP (cAMP). This massive accumulation of intracellular cAMP causes fluid efflux into the extracellular space (edema) and suppresses neutrophil function.

Step-by-Step Solution

1
Identify the clinical syndrome and pathogen from the vignette.
The patient's occupation (leather processing), rapid progression from flu-like symptoms to shock, widened mediastinum on chest X-ray, and blood cultures showing boxcar-shaped Gram-positive bacilli indicate inhalational anthrax caused by Bacillus anthracis.
Recognizing the classic presentation of inhalational anthrax is necessary to determine the relevant virulence factors.
2
Recall the components of the Bacillus anthracis exotoxin.
The toxin is tripartite, consisting of Protective Antigen (PA), Edema Factor (EF), and Lethal Factor (LF).
The question specifically asks for the mechanism of the component responsible for profound tissue edema.
3
Determine the specific biochemical mechanism of Edema Factor (EF).
Edema Factor is an adenylate cyclase that directly synthesizes cyclic AMP (cAMP) but is uniquely dependent on the presence of host intracellular calmodulin for its activation.
Differentiating the mechanism of EF from other cAMP-increasing toxins (like Pertussis or Cholera toxins) isolates the correct choice.

Key Concept

The pathophysiology of inhalational anthrax depends on a tripartite exotoxin where Edema Factor (EF), a calmodulin-dependent adenylate cyclase, is directly responsible for massive fluid accumulation and immune evasion.
Estimated Time:1m 30s
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Infectious Disease Mechanisms

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