True about exotoxin –
**Core Concept**
Exotoxins are potent toxins produced by certain bacteria, such as Clostridium tetani, Clostridium botulinum, and Corynebacterium diphtheriae. These toxins are secreted outside the bacterial cell and can cause severe disease in humans.
**Why the Correct Answer is Right**
Exotoxins work by binding to specific receptors on host cells, triggering a cascade of intracellular signaling events that ultimately lead to cell death or dysfunction. For example, tetanus toxin blocks the release of neurotransmitters at the neuromuscular junction, causing muscle rigidity and spasms. Diphtheria toxin inhibits protein synthesis by cleaving the elongation factor 2, leading to cell death.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because endotoxins are lipopolysaccharides found in the outer membrane of Gram-negative bacteria, not exotoxins.
**Option B:** Incorrect because while toxins can be produced by fungi, exotoxins are specifically associated with bacterial pathogens.
**Option C:** Incorrect because while some toxins may be produced in response to environmental stimuli, exotoxins are typically constitutively produced by certain bacteria.
**Clinical Pearl / High-Yield Fact**
Exotoxins can be neutralized by specific antibodies, and vaccination is an effective strategy for preventing disease caused by these toxins. The development of exotoxin-based vaccines has been a major public health achievement, protecting millions of people worldwide from diseases such as tetanus and diphtheria.
**Correct Answer:** D.