Introduction

Matching bacterial toxins such as cholera toxin, diphtheria toxin, lipopolysaccharide and tetanus toxin with the correct functional categories is a high‑yield topic in microbiology MCQs for CSIR NET and other competitive exams. Understanding which toxins act as enterotoxin, cytotoxin, endotoxin or neurotoxin helps students quickly eliminate wrong options and strengthens conceptual clarity in medical microbiology.​

Step‑by‑step matching logic

Group I:
P) Cholera toxin
Q) Diphtheria toxin
R) Lipopolysaccharide
S) Tetanus toxin

Group II:

1. Endotoxin

2. Neurotoxin

3. Enterotoxin

4. Cytotoxin

P) Cholera toxin → Enterotoxin (3)

• Cholera toxin from Vibrio cholerae specifically targets intestinal epithelial cells, massively raising intracellular cAMP and causing secretion of chloride and water into the gut lumen.​

• Because it primarily acts on the intestine to induce profuse watery diarrhoea, cholera toxin is classified as an enterotoxin, so P matches with 3.​

Q) Diphtheria toxin → Cytotoxin (4)

• Diphtheria toxin is an A‑B exotoxin produced by toxigenic Corynebacterium diphtheriae that enters host cells and ADP‑ribosylates eukaryotic elongation factor‑2, shutting down protein synthesis.​

• This inhibition causes death of host cells (not selective for nerves or gut), so it is best described as a cytotoxin, giving Q–4.​

R) Lipopolysaccharide → Endotoxin (1)

• Lipopolysaccharide (LPS) is a major component of the outer membrane of Gram‑negative bacteria; its lipid A portion is a potent endotoxin that triggers strong inflammatory responses and can lead to septic shock.​

• Hence lipopolysaccharide is correctly matched with endotoxin, so R–1.​

S) Tetanus toxin → Neurotoxin (2)

• Tetanus toxin (TeNT) from Clostridium tetani is a powerful protein neurotoxin that is taken up at neuromuscular junctions and transported to the CNS, where it blocks release of inhibitory neurotransmitters like glycine and GABA.​

• Because it acts specifically on nerve cells and interferes with neurotransmission, it is classified as a neurotoxin, so S–2.​

Putting these together: P–3, Q–4, R–1, S–2 → option (C).

Option‑wise analysis

Option (A): P–1, Q–2, R–3, S–4

• This option labels cholera toxin as an endotoxin and lipopolysaccharide as an enterotoxin, which reverses the well‑established roles of LPS as endotoxin and cholera toxin as enterotoxin.​

• It also incorrectly treats diphtheria toxin as a neurotoxin and tetanus toxin as a cytotoxin, ignoring the specific neurotoxic action of tetanus neurotoxin.​

Option (B): P–3, Q–2, R–1, S–4

• Cholera toxin (P–3) and lipopolysaccharide (R–1) are correct here, but diphtheria toxin is wrongly categorised as a neurotoxin and tetanus toxin as a cytotoxin.​

• Since only two matches are correct, this option must be rejected in a single‑correct MCQ.​

Option (C): P–3, Q–4, R–1, S–2 ✅

• All four pairings here agree with standard microbiology: cholera toxin as enterotoxin, diphtheria toxin as cytotoxin, LPS as endotoxin, and tetanus toxin as neurotoxin.​

• Therefore option (C) is the only fully correct choice.

Option (D): P–4, Q–1, R–2, S–3

• This option wrongly assigns cholera toxin as cytotoxin and tetanus toxin as enterotoxin, which contradicts their classic intestinal and neural targets respectively.​

• It also mislabels diphtheria toxin as endotoxin and LPS as neurotoxin, reversing textbook definitions of exotoxin versus endotoxin.​

Key exam takeaway

• Enterotoxin: acts on intestinal mucosa, e.g. cholera toxin causing secretory diarrhoea.​

• Cytotoxin: damages or kills host cells in general, e.g. diphtheria toxin inhibiting protein synthesis.​

• Endotoxin: structural LPS of Gram‑negative bacteria, especially lipid A, responsible for septic shock and fever.​

• Neurotoxin: targets neurons, e.g. tetanus toxin blocking inhibitory neurotransmitter release and causing spastic paralysis.​