Serum vitamin A levels
Antinuclear antibody titer
Answer: A) Serum vitamin A levels
Explanation: The image shows a Bitot spot. Bitot spots are caused by vitamin A deficiency. They represent keratinization of the conjunctiva along with colonization with Corynebacterium xerosis. Vitamin A replacement is necessary to prevent the consequences of vitamin deficiency, including xerophthalmia, keratomalacia, nyctalopia, and blindness. Vitamin A deficiency mostly affects children in the developing world and may be fatal. Rheumatoid factor, thyroid levels, and antinuclear antibodies will not explain the conjunctival findings.