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: Physiology
[{'content': 'Sodium channels are specifically blocked by: \nSelect and explain. \nA. Nifedipine, B. Tetrodotoxin, C. Tetraethyl lead, D. Choline', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Another means for studying the flow of ions through an individual type of channel is to block one type of channel at a time. For instance, the sodium channels can be blocked by\xa0a\xa0toxin\xa0called\xa0tetrodotoxin\xa0when\xa0 it\xa0 is\xa0 applied\xa0 to\xa0 the outside of the cell membrane where the sodium activation gates\xa0are\xa0located. Conversely, tetraethylammonium ion blocks the potassium channels when it is applied to the interior of the nerve fiber.\nKey Concept:\nTetrodotoxin (TTX), one of the most potent poisons known, speciﬁcally blocks the Na+ channel. TTX binds to the extracellular side of the sodium channel.\nReference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION(INTERNATIONAL EDITION ) page no 67', 'role': 'assistant'}]