Which of the following statements are true about diphtheria toxin? A. It inhibits EF-2 B. It is an A-B type exotoxin C. It has particular affinity for myocardial and nervous tissue D. Toxin is produced only by strains carrying a plasmid Choose the correct answer:

Appeared in: INI-CET EXAM -2026

Explanation

Diphtheria toxin is a classic A-B exotoxin. The B (Binding) subunit attaches to host cell receptors, facilitating the entry of the A (Active) subunit.
The A subunit is an enzyme that catalyzes the ADP-ribosylation of Elongation Factor-2 (EF-2), a critical component in host cell protein synthesis. This inactivation halts protein production and causes cell death.
While the bacterial infection is typically localized to the respiratory tract, the toxin enters the bloodstream and has a high affinity for myocardial (heart muscle) and nervous tissues, leading to potentially fatal complications like myocarditis and polyneuropathy.

Option A: This option is incomplete. While statements A and C are true, it incorrectly omits statement B. Diphtheria toxin being an A-B type exotoxin is a fundamental characteristic.
Option C: This option is incorrect because statement D is false. The gene responsible for diphtheria toxin production is not located on a plasmid.
Option D: This option is incorrect because it includes the false statement D (plasmid-borne toxin) and omits the true statements A (inhibits EF-2) and C (affinity for heart/nerve tissue).

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Mechanism and properties of Diphtheria Toxin as background academic context rather than a clinical decision trigger.
The local action of the toxin in the pharynx causes tissue necrosis and the formation of a thick, gray pseudomembrane, a hallmark sign of respiratory diphtheria that can cause airway obstruction.
Understanding the toxin's affinity for cardiac and nervous tissue is crucial for monitoring patients for life-threatening complications like myocarditis (which can cause heart failure and arrhythmias) and neuropathy (which can lead to paralysis).
Prevention through vaccination is key. The DTaP/Tdap vaccine contains diphtheria toxoid (an inactivated toxin), which stimulates the body to produce neutralizing antibodies, providing protection against the toxin's effects.

How to Approach the Question

First, identify the core subject of the question, which is the characteristics of diphtheria toxin.
Next, systematically evaluate each of the four statements (A, B, C, D) for its scientific accuracy based on your knowledge of microbiology and pathology.
Mentally label each statement as 'True' or 'False'. For example: A is True (inhibits EF-2), B is True (A-B toxin), C is True (affects heart/nerves), D is False (it's phage-encoded, not plasmid).
After evaluating all statements, examine the answer options (A, B, C, D), which are combinations of the statements.
Select the option that correctly groups the statements you identified as true. In this case, option B correctly identifies A, B, and C as the true statements.

Concept Tested & Keywords

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E6 Pathology- ROBBINS & COTRAN PATHOLOGIC BASIS OF DISEASE 10TH Ed pp. 368-370, 369-371

E6 Medicine Harrison 22e Part 1 p. 1015-1017

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Attempt every question from this paper in a timed mock, then review the full solution for each one.