NBME 22 Answers : Block 1 : #50 = A 9-year-old boy is brought to the physician ...

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submitted by organicmechanic(2),

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ls3076 why not membrane receptor? +6

a1913 delF508 is a 3 base pair deletion of phenylalanine at amino acid position 508. Mutation causes impaired post-translational processing of CFTR (improper folding) which rough ER detects. Sends mutant misfolded CFTR to the proteasome for degradation, preventing it from reaching cell surface. So problem is not malfunctioning CFTR channels in the surface; problem is complete absence of CFTR on cell surface (since they keep getting misfolded and sent to proteasome to be trashed). Source of primary problem: error in protein structure +6

angelaq11 @Is3076 because the CFTR is a channel not a receptor. +16

rainlad FA 2019 p. 60 +

dysdiadochokinesia @a1913 is correct- as for @angelaq11, you can still have a receptor that also functions as a channel as they are not mutually exclusive. An example of this is the nAChR found on postsynaptic NMJ neurons. This is a non-selective, ligand-gated, ionotropic receptor that functions as a channel once its ligand (i.e., ACh) has bound to the active site to induce conformational change. Similarly on the same realm: CFTR is an ionotropic receptor that concurrently functions as a Cl- channel once its ligands (ie. 2 ATP) is bound to open the channel and enable Cl- flux. This question in particular is asking for the underlying pathophysiologic mechanism for cystic fibrosis, which boils down to an issue with the primary structure of a protein resulting in its misfolding and subsequent sequestration/degradation. +1