Okay, let's tackle this question about diphtheria toxin. The question is asking which other toxin it resembles. First, I need to recall what diphtheria toxin does. Diphtheria toxin is produced by Corynebacterium diphtheriae. It works by inhibiting protein synthesis. The toxin ADP-ribosylates elongation factor 2 (EF-2), which stops protein synthesis in eukaryotic cells.

Now, thinking about other toxins that affect protein synthesis similarly. The big ones are from other bacteria. For example, Pseudomonas aeruginosa produces exotoxin A, which also ADP-ribosylates EF-2. So both toxins target the same elongation factor, leading to inhibition of protein synthesis. That would make them similar in mechanism.

Let me check the other options. If the options include tetanus toxin or botulinum toxin, those work differently. They affect neurotransmitter release by cleaving SNARE proteins. So they're not similar. Cholera toxin and E. coli heat-labile toxin act on G proteins, causing cyclic AMP issues. Not the same as diphtheria. Anthrax toxin has different components and mechanisms. So the only one that matches is exotoxin A from Pseudomonas.

Clinical pearl: Remember that both diphtheria toxin and Pseudomonas exotoxin A inhibit protein synthesis via EF-2 ADP-ribosylation. This is a key distinction from other toxins that act on different pathways. So the correct answer should be the one that refers to Pseudomonas exotoxin A.

**Core Concept** Diphtheria toxin and Pseudomonas exotoxin A both inhibit protein synthesis in eukaryotic cells by ADP-ribosylating elongation factor 2 (EF-2), a key step in translation. This mechanism distinguishes them from other bacterial toxins that target different pathways.

**Why the Correct Answer is Right** Diphtheria toxin catalyzes the transfer of ADP-ribose from NAD+ to a diphthamide residue on EF-2, rendering it inactive. Pseudomonas exotoxin A follows the same enzymatic mechanism, irreversibly inactivating EF-2. Both toxins are produced by Gram-positive bacteria and require host cell uptake via receptor-mediated endocytosis for activity.

**Why Each Wrong Option is Incorrect**
**Option A:** Tetanus toxin (Clostridium tetani) cleaves SNARE proteins, blocking inhibitory neurotransmission.
**Option B:** Cholera toxin (Vibrio cholerae) ADP-ribosylates Gα subunits of Gs proteins, increasing cAMP.
**Option C:** Anthrax toxin (Bacillus anthracis) components (edema factor, lethal factor) act on different pathways, including calmodulin-dependent adenylyl cyclase.

**Clinical Pearl / High-Yield Fact** Remember "EF-2 inactivation" as the hallmark of diphtheria and Pseudomonas exotoxin A. Contrast with cholera toxin’s G protein targeting and tetanus/botulinum toxins’ SNARE cleavage. This distinction is critical for NEET PG/USMLE toxin mechanism questions.

**Correct Answer: B. Pseudomonas exotoxin A

✓ Correct Answer: C. Snake