Neuroscience MCAT Practice Question
A 68-year-old man presents with progressive vision loss, particularly difficulty with night vision and peripheral visual field defects. Genetic testing reveals a mutation in the CNGA3 gene, which encodes the alpha subunit of cyclic nucleotide-gated (CNG) ion channels in retinal photoreceptors. His ophthalmologic examination shows cone dystrophy with relative sparing of rod function. Which of the following best explains why this mutation primarily affects cone vision rather than rod vision in this patient?
Answer choices
- ARod photoreceptors do not express cyclic nucleotide-gated channels in their outer segments
- BCNGA3 is the primary CNG channel subunit in cones, while rods express different CNG subunits with redundant functionCorrect answer
- CCones require phosphodiesterase activation to close CNG channels, whereas rods use voltage-gated calcium channels instead
- DRod bipolar cells express glutamate receptors insensitive to photoreceptor-derived glutamate in this genetic background
- ECone outer segments have higher basal cGMP levels that compensate for reduced channel function
- FCNGA3 mutations preferentially disrupt the G-protein cascade in cone cells due to higher opsin expression
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