Examkin

Soru

Zorluk: OrtaInfectious Disease Mechanisms

A 28-year-old aid worker who recently returned from a disaster relief mission in a developing country presents to the emergency department with profuse, "rice-water" diarrhea and severe leg cramps. His blood pressure is 80/50 mm Hg, and his pulse is 125/min. Physical examination reveals deeply sunken eyes, dry mucous membranes, and delayed capillary refill. He is rapidly resuscitated with intravenous lactated Ringer's solution. Stool cultures subsequently grow curved, Gram-negative, oxidase-positive rods. The principal virulence factor of this pathogen causes the patient's symptoms through a precise, multi-step intracellular mechanism.

Arrange the following cellular events in the correct chronological order to describe the pathogenesis of this organism's primary exotoxin.

  1. 1Binding of the pentameric B subunit to GM1 gangliosides on the cell surface
  2. 2Receptor-mediated endocytosis and retrograde transport to the endoplasmic reticulum
  3. 3Separation and retrotranslocation of the active A1 subunit into the cytosol
  4. 4ADP-ribosylation of the stimulatory G-protein alpha subunit (Gs-alpha)
  5. 5Continuous activation of adenylate cyclase leading to cyclic AMP accumulation

Cevap

The correct chronological order of cholera toxin pathogenesis is: Binding of the pentameric B subunit to GM1 gangliosides → Receptor-mediated endocytosis and retrograde transport to the endoplasmic reticulum → Separation and retrotranslocation of the active A1 subunit into the cytosol → ADP-ribosylation of the stimulatory G-protein alpha subunit (Gs-alpha) → Continuous activation of adenylate cyclase leading to cyclic AMP accumulation.
The clinical presentation of profuse 'rice-water' diarrhea in a returning traveler with oxidase-positive, curved Gram-negative rods is pathognomonic for Vibrio cholerae. Cholera toxin is an AB5 enterotoxin that exerts its effects through a specific intracellular cascade. First, the pentameric B subunit binds to GM1 ganglioside receptors on the apical surface of enterocytes. The toxin is then internalized via receptor-mediated endocytosis and travels via retrograde transport through the Golgi apparatus to the endoplasmic reticulum (ER). In the ER, the catalytic A subunit is cleaved and unfolded, allowing the A1 fragment to retrotranslocate into the cytosol. Once in the cytosol, the A1 fragment acts as an enzyme to ADP-ribosylate the stimulatory G-protein alpha subunit (Gs-alpha). This covalent modification locks Gs-alpha in its active, GTP-bound state. The persistently active Gs-alpha then continuously stimulates adenylate cyclase, leading to massive intracellular accumulation of cyclic AMP (cAMP). High cAMP levels activate protein kinase A (PKA), which phosphorylates and opens the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) chloride channels, resulting in massive secretory diarrhea.

Adım Adım Çözüm

1
Identify the causative organism based on the clinical vignette.
The patient's 'rice-water' diarrhea, severe dehydration, and stool cultures growing curved, oxidase-positive, Gram-negative rods indicate infection with Vibrio cholerae.
Accurate identification of the pathogen is necessary to determine the specific virulence factors and molecular mechanisms involved.
2
Determine the primary virulence factor responsible for the patient's symptoms.
The symptoms are caused by cholera enterotoxin, an AB5 exotoxin.
Understanding the structure and function of the AB5 toxin is the foundation for analyzing its cellular entry and mechanism of action.
3
Sequence the events from cell surface binding to the ultimate physiological effect.
The sequence is: receptor binding (GM1) → endocytosis and retrograde transport → A1 subunit release into cytosol → ADP-ribosylation of Gs-alpha → adenylate cyclase activation and cAMP accumulation.
This specific pathophysiological cascade explains the massive fluid efflux (secretory diarrhea) seen in the clinical presentation.

Anahtar Kavram

The multi-step intracellular pathogenesis of cholera toxin, an AB5 enterotoxin that causes secretory diarrhea via ADP-ribosylation of Gs-alpha and subsequent cAMP accumulation.
Bu soruyu puanla

Konular

Infectious Disease Mechanisms

Bu konudan daha fazla soru