**Core Concept:** Retinitis pigmentosa (RP) is a group of inherited retinal disorders characterized by progressive degeneration of rod photoreceptor cells, leading to night blindness and loss of peripheral vision. The primary pathophysiology involves dysfunction or death of rod cells, which are responsible for low light vision.

**Why the Correct Answer is Right:** RP is primarily caused by mutations in genes involved in the photoreceptor structure and function, such as RP1, RP2, and rhodopsin. These genetic defects lead to impaired function or death of rod cells, causing the characteristic symptoms and visual loss.

**Why Each Wrong Option is Incorrect:**

A. RP is not associated with cataract formation since cataracts are related to the aging process and exposure to UV radiation, while RP is a genetic disorder affecting photoreceptor cells.

B. RP is not caused by a deficiency of vitamin A, as vitamin A deficiency primarily affects night vision but does not cause the selective loss of rod cells.

C. RP is not caused by a defect in the optic nerve, as optic nerve dysfunction would lead to generalized vision loss rather than selective rod cell degeneration.

D. RP is not caused by a hormonal imbalance, as hormonal disturbances typically affect the entire visual system, leading to more generalized visual impairment.

**Clinical Pearl:** RP is an inherited disorder with a wide spectrum of clinical presentation. While it primarily affects rod cells and leads to night blindness, it can also impact cone cells, contributing to visual field constriction and reduced color vision. Early intervention and genetic counseling are essential for affected individuals and their families to manage the disease and plan for future pregnancies.