In retinitis pigmentosa, "night-blindness" is an early symptom of visual loss. Which of the following explains this phenomenon?

Retinitis pigmentosa is a familial degenerative disease of the retina that most often has recessive genetics. The initial problem appears to be alterations in the pigmented epithelium below the retina, paicularly in the most anterior poions of the retina. The pigmented epithelial cells become disrupted and leak pigment, which accumulates along the attenuated blood vessels (and can be seen with an ophthalmoscope). A consequence of this damage is that the rod cells that are normally nutritionally suppoed and "groomed" by the pigment epithelium also undergo degenerative changes. Since the cones are relatively preserved, day vision is preserved, but night vision, which is highly dependent on rods, begins to decay. With disease progression, vision is completely or nearly completely lost and the retina becomes so distoed that only a single row of cone nuclei with scattered stumpy cone remnants is all that remains of the photoreceptor layer. Ref: Fletcher E.C., Chong N., Augsburger J.J., Correa Z.M. (2011). Chapter 10. Retina. In P. Riordan-Eva, E.T. Cunningham, Jr. (Eds), Vaughan & Asbury's General Ophthalmology, 18e.