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
Receptor for diphtheria toxin lies at ? 
Select and explain. 
A. Cell membrane, B. Mucous membrane, C. Nucleus, D. None
A: Cell membrane
Answer: A: Cell membrane 
Explanation: Ans. is 'a' i.e., Cell membrane 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. The toxin binds to specific receptors (HB-EGF receptors) present on susceptible cells and enters by receptor-mediated endocytosis. Toxin has special affinity for ceain tissues such as myocardium, adrenals and nerve endings.