Respiratory CAS Practice Exam

Elongation factor-2 (EF-2) is essential for the translocation step of protein synthesis; when it is ADP-ribosylated, EF-2 is inactivated and protein synthesis stops, leading to cell death. Exotoxin A from Pseudomonas aeruginosa uses this exact mechanism to inhibit EF-2, making it the toxin described in the stem. The other toxins act by different routes: Shiga toxin depurinates rRNA at the ribosome; cholera toxin alters Gs to raise cAMP and trigger secretion; diphtheria toxin also ADP-ribosylates EF-2 but is produced by Corynebacterium diphtheriae, not Pseudomonas. So Exotoxin A best fits both the mechanism and the organism described.