This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vitreous and retina, Subject name: Ophthalmology
Which of the parameter is decreased in Retinitis pigmentosa 
Select and explain. 
A. Arachidonic acid, B. Docosahexanoic acid, C. Trielonic acid, D. Thromboxane
B: Docosahexanoic acid
Answer: B: Docosahexanoic acid 
Explanation: * Retinitis Pigmentosa is a familial disease characterized by degeneration of photoreceptor with progressive night blindness and constriction of peripheral vision (may lead to functional blindness) * Docosahexanoic acid (DHA) comprises a small percentage (1-4%) of the total fatty acids of the membranes of most human tissues; however, it accounts for 30-40% of fatty acids in rod photoreceptor outer segments of the human retina. * Many patients with RP have lower plasma and red blood cell (RBC) lipid levels of the polyunsaturated fatty acid docosahexaenoic acid than non-affected individuals. The majority of patients with XLRP had 30-40% lower DHA levels in RBC lipids than normally sighted controls RP TRIAD:- * Includes fundus findings: Waxy pallor of the disc, aeriolar attenuation and bony corpuscular pigment clumps at the mid - peripheral retina.