## **Core Concept**
Retinitis pigmentosa (RP) is a group of genetic disorders affecting the retina's ability to respond to light, leading to progressive vision loss. It primarily affects the rod cells, which are responsible for peripheral and night vision. The condition results from mutations in genes that encode proteins crucial for photoreceptor function and survival.

## **Why the Correct Answer is Right**
The correct answer, **night blindness**, is the earliest symptom of retinitis pigmentosa because RP initially affects the rod cells. Rod cells are more numerous in the peripheral retina and are responsible for vision in low light conditions. Damage to these cells leads to difficulty seeing in the dark or at night, which is known as night blindness. This symptom appears before significant daytime vision problems because cone cells, which mediate daytime vision, are initially spared.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While visual field defects can occur in RP, they typically present after night blindness. Patients may initially experience loss of peripheral vision, but this is not the earliest symptom.
- **Option B:** Decreased visual acuity is a later symptom of RP. It occurs as the disease progresses and affects the macula or the central part of the retina where cone cells are concentrated.
- **Option C:** (Assuming this is another option) Without the specific text, it's challenging to address directly, but any option suggesting a symptom that appears after night blindness (like significant visual field loss or decreased visual acuity) would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that retinitis pigmentosa often presents with **night blindness** as its earliest symptom. This symptom can sometimes be reported by patients in their teenage years or even earlier. A classic clinical correlation is the association of RP with **inverse square sign** or **tunnel vision** in advanced stages, but night blindness is the initial clue.

## **Correct Answer: B. night blindness**