![Q.78. Determine the correctness or otherwise of the following Assertion [a] and the Reason [r]. Assertion [a]: Diphtheria exotoxin is an example of A-B type toxin. Reason [r]: The A component of the toxin is released from the host cell while the B component inhibits protein synthesis and kills the host cell. (A) Both [a] and [r] are true and [r] is the correct reason for [a] (B) Both [a] and [r] are true but [r] is not the correct reason for [a] (C) Both [a] and [r] are false (D) [a] is true but [r] is false](https://www.letstalkacademy.com/wp-content/uploads/2026/01/slide_78.png)
Q.78. Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].
Assertion [a]: Diphtheria exotoxin is an example of A-B type toxin.
Reason [r]: The A component of the toxin is released from the host cell while the B component inhibits
protein synthesis and kills the host cell.
(A) Both [a] and [r] are true and [r] is the correct reason for [a]
(B) Both [a] and [r] are true but [r] is not the correct reason for [a]
(C) Both [a] and [r] are false
(D) [a] is true but [r] is false
Diphtheria exotoxin exemplifies A-B type toxins in microbiology. The assertion-reason question tests understanding of its mechanism, with option (D) as the correct answer.
Assertion and Reason Breakdown
Assertion [a] states diphtheria exotoxin is an A-B type toxin, which holds true. Diphtheria toxin, produced by Corynebacterium diphtheriae, consists of A subunit (enzymatic) and B subunit (binding).
Reason [r] claims the A component releases from the host cell while B inhibits protein synthesis, but this reverses roles. The B subunit binds host receptors and stays in the endosome; A translocates to cytosol and ADP-ribosylates eEF-2 to halt protein synthesis.
Mechanism of Action
The toxin enters via endocytosis after B binds cell receptors. Acidification cleaves and releases A into cytosol, where it inactivates elongation factor 2, stopping translation and killing cells.
B forms pores for A translocation but does not enter cytosol or inhibit synthesis.
Options Analysis
| Option | Description | Correctness |
|---|---|---|
| (A) | Both true, [r] correct reason for [a] | Incorrect: [r] false |
| (B) | Both true, [r] not reason for [a] | Incorrect: [r] false |
| (C) | Both false | Incorrect: [a] true |
| (D) | [a] true, [r] false | Correct: Matches facts |
This structure aids exam prep in molecular biology and bacterial toxins.
