Among the classic bacterial exotoxins of medical importance, which toxin is considered the most potent on a per-weight basis? Choose the toxin widely cited as the most toxic known to humans.

Introduction:
Bacterial exotoxins differ in mechanism and potency. Recognizing the most potent exotoxin helps in understanding pathogenesis, public health risk, and laboratory biosafety. The benchmark for extreme potency is botulinum neurotoxin produced by Clostridium botulinum.

Given Data / Assumptions:

• We compare tetanus, diphtheria, botulinum, cholera, and Shiga toxins.
• “Most potent” refers to minimal lethal dose in humans or experimental animals on a weight basis.
• No calculation is required; this is a concept-recall item.

Concept / Approach:
Botulinum toxin blocks acetylcholine release at neuromuscular junctions, causing flaccid paralysis. Its estimated lethal dose is extraordinarily low compared with other toxins, which is why food safety protocols for canning are stringent.

Step-by-Step Solution:
Step 1: Recall the relative toxicities of major exotoxins taught in standard texts. Step 2: Recognize botulinum neurotoxin as the extreme outlier in potency. Step 3: Select “Botulinum toxin.”

Verification / Alternative check:
Outbreak reviews and toxin LD50 data consistently place botulinum neurotoxin as the most potent biologic toxin by weight. Clinical management centers on antitoxin and supportive care (especially ventilatory support).

Why Other Options Are Wrong:
Tetanus toxin – highly potent but not above botulinum on a weight basis.
Diphtheria toxin – inhibits protein synthesis but is less potent than botulinum toxin.
Cholera toxin – causes secretory diarrhea; potency per weight is far lower.
Shiga toxin – cytotoxic; still less potent than botulinum neurotoxin.

Common Pitfalls:
Confusing clinical severity or mortality rate with intrinsic toxin potency; potency here is a biochemical metric, not case fatality under modern care.

Final Answer:
Botulinum toxin.