Cobra venom is competitive inhibitor of Ach. What would you expect to happen when someone get bit.
A. Excessive stimulation of most parasympathetic neurons
B. CNS neuronal degeneration and death
C. Respiratory muscle paralysis
D. rapid heart rate and dehydration
I know A /B is sure wrong
C I have no clue
D I reason to be right because Ach is parasympathic on the heart which slow heart down so if you inhibit Ach you increase heart rate.
However The books says the answer is D while their explanation is kinda crappy. can someone confirm this?
This question is getting at the sympathetic and parasympathetic activation, but you are right C and D could both be considered correct, but you have to assume they are referring to the diaphragm in C, which they didn't specify in the answer choices, so D would be the most direct and correct answer.
To go through 1 by 1.
A) Incorrect because an antagonist would cause a REDUCTION in the activation of parasympathetic neurons
B) Is incorrect for a number of reasons, 1) antagonist most likely won't cross the blood/brain barrier 2) A competitive antagonist is usually not strong enough to cause death, because you still get activation, it's just greatly reduced. Plus a number of other reasons I won't get into, because they aren't really that important based on the question.
C) I'm going to go out on a limb on this one and say they are referring to the diaphragm, which would experience some paralysis, so this question leaves a little ambiguity. Because if you were give someone an antagonist to ACh such as Curare (a nicotinic receptor antagonist), this would react at the muscles and cause paralysis of the diaphragm. That would be respiratory muscle paralysis. Now the question is if they are referring to nicotinic or muscarinic receptors. I think this is a possibility for an answer, but since the venom could inhibit multiple systems depending on the receptor preference it does leave some abiguity for it being it being the correct answer.
D) This one is correct under all circumstances, because, like you said, if you give someone a competitive inhibitor of ACh, then an immediate expected response would be an increase in heart rate, because ACh released by the vagus nerve causes a decrease in heart rate, so when you inhib that process the heart rate will increase because it can no longer regulate itself. The dehydration is caused by the decreased responsiveness of the parasympathetic system, which would lead to activated sympathetic responses compared to the regular equilibrium between the systems. This is because all of the effectors from the parasympathetic system utilize ACh as a primary NT, so when you block/reduce that activate you in turn reduce the efficacy of that system.
Overall, the ambiguity of C with no abiguity of D makes D a better answer.
I have seen a couple AAMC questions that use similar logic, which is the BEST answer. Some of them will be completely wrong, but you might have two that are very close but one is generally a better answer than the other.
They are usually less ambigous than the above question though.
