This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Retina, Subject name: Ophthalmology
The most constant and critical finding in retinitis pigmentosa is 
Select and explain. 
A. Bone spicule pigmentation in retina, B. Significantly reduced ERG, C. Tubular visual fields, D. Abnormality in rhodopsin gene
B: Significantly reduced ERG
Answer: B: Significantly reduced ERG 
Explanation: Typical electrophysiological changes in retinitis pigmentosa appear early in the disease before the subjective symptoms or the objective signs (fundus changes) appear and are most constant and critical finding. Electroretinogram (ERG) is initially subnormal (scotopic affected before photopic; b-waveaffected before ;a wave) and eventually extinguished. Electro-oculogram {EOG) is subnormal with an absence of light peak. Bony spicules are not seen in all cases. Tubular visual field can occur due to other causes as well like open angle glaucoma.