Ans. b. Oguchi's disease Mizuo phenomenon is seen in Oguchi disease."In Oguchis disease the characteristic golden yellowish metallic sheen (color) of posterior pole (fundus) in light adapted state reverts to normal after prolonged dark adaptation, a phenomenon described by and named after Japanese ophthalmologist Mizuo Re-exposure to light results in the return of golden yellow metallic sheen.** Female carriers of X-linked (XL) retinitis pigmentosa may have normal fundi or show a golden metallic (tapetal) reflex at macula and/or small peripheral patches of hone spicule pigmentation. The inverse Mizuo phenomenon seen in XL-RP is characterized by appearance of golden sheen in dark (hut no golden sheen in light)."Oguchi's Disease* Oguchi's disease is a rare autosomal recessive trait characterized by congenital stationary night blindness and a unique morphological and functional abnormality of the retina.* Show Mizuo phenomenon^* Mizuo phenomenon: Fundus is golden yellow in light adapted state which becomes normal after prolonged dark adaptation. Reexposure to light results in the return of the metallic y ellow sheeny.* Rod function is absent after 30-minutes of dark adaptation but recovers to a near normal level after a long period of dark adaptation (i.e. -- normal rod thresholds reached only after 4 hours or longer instead of normal 30 minutes).Etiopathogenesis:* A null allele in genes for each of rhodopsin kinase and arrestin (proteins responsible for terminating phototransduction cascade) is responsible for Oguchi's disease.* Therefore, the persistent low level of light may desensitize the rods continually.* Mizuo phenomenon suggest an abnormality in rod pigment, rhodopsin.* Cone function is normal because cone adaptation, final cone thresholds and photopicERG response are normal.* Rod function is abnormal with delayed dark adaptation and scotopic ERG showing only a small electronegative response, even when the rod threshold have reached normal.Clinical Features:* Patients have non-progressive night blindness since young childhood writh normal day vision, but they often claim improvement of light sensitivities w hen they remain long in the dark environment* The fundus oculi presents a most peculiar appearance.