Concept: This question tests your knowledge of the structure and function of the different layers of the human eyeball, specifically focusing on the vascular and pigmented layer.

Why (A) is correct: The choroid is the middle layer of the eyeball. It is richly supplied with blood vessels that provide nourishment to the retina. The choroid also contains numerous pigment cells (melanocytes) which give it a dark, bluish color, preventing internal reflection of light within the eye.

Why other options are wrong:

• (B) Sclera: The sclera is the outermost, tough, white fibrous layer; it is avascular.
• (C) Retina: The retina is the innermost, light-sensitive layer containing photoreceptor cells; it is not primarily blue in color and its blood supply comes from the choroid and retinal vessels.
• (D) Cornea: The cornea is the transparent front part of the sclera; it is avascular.

Common Mistake: Students sometimes confuse the functions of the choroid and retina, or forget which layer is vascularized.

NEET Tip / Mnemonic: Remember the layers from outside to inside: Sclera, Choroid, Retina (SCR). The 'C' in Choroid can remind you of 'Color' (blue) and 'Circulation' (blood vessels).

Correct Answer: (A)