**Core Concept:** Night blindness and tubular vision are symptoms commonly associated with optic neuritis, a condition where the optic nerve is inflamed. The optic nerve is responsible for transmitting visual information from the retina to the brain. Inflammation can lead to axonal damage, affecting the transmission of visual signals.

**Why the Correct Answer is Right:** The correct answer is **D.** Optic neuritis due to multiple sclerosis (MS) is the most likely diagnosis. MS is an autoimmune disease that affects the central nervous system, including the optic nerve. Inflammation and demyelination in MS can cause optic neuritis, leading to symptoms like night blindness, tubular vision, and visual field defects (ring scotomas on perimetry).

**Why Each Wrong Option is Incorrect:**

A. Night blindness and tubular vision can also result from other conditions, such as vitamin A deficiency or inherited retinal diseases, but these are less likely in a young adult.
B. Vitamin A deficiency is unlikely due to normal anterior segment findings and absence of ocular structural abnormalities on fundoscopy.
C. Vitamin A deficiency causes optic atrophy, not inflammation. Optic neuritis is a more appropriate diagnosis for the given symptoms and findings.
D. Multiple sclerosis is the most plausible diagnosis considering the young age, optic neuritis, and associated visual field defects.

**Clinical Pearl:**

The presence of bony corpuscles like spicules of pigmentation in the mid-peripheral retina indicates the classic "salmon patch" sign, which is characteristic of optic neuritis due to MS. This sign is not seen in other conditions.

**Correct Answer:** D. Multiple Sclerosis