This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
True about diphtheria toxin ? 
Select and explain. 
A. Heat stable, B. Acts through cGMP, C. Consists of three fragments, D. Special affinity for brain
A: Heat stable
Answer: A: Heat stable 
Explanation: Ans. is 'a' i.e., Heat stable "Diphtheria toxin is a heat-stable polypeptide, composed of two fragments" - Medical microbiology Diphtheria toxin The diphtheria toxin acts by inhibiting protein synthesis. It inhibits polypeptide chain elongation in the presence of nicotinamide adinine dinucleotide (NAD) by inactivating elongation factor, EF - 2. The diphtheria toxin is a protein which consists of two fragments, A and B. Both fragments are necessary for the toxic effect : Fragment A - has enzymatic activity and inhibits protein synthesis by inhibiting the chain elongation by inactivating the elongation factor - 2 (Ef - 2) Fragment B - responsible for binding the toxin to the cells. Toxin has special affinity for ceain tissues such as myocardium, adrenals and nerve endings. The strain almost universally used for toxin production is the "Park williams 8 strain".