Final answer:
The most appropriate initial step in managing this patient's yellow and painful big toe is KOH examination of nail scrapings.
Step-by-step explanation:
The most appropriate initial step in managing this patient with a yellow and painful big toe on her right foot is B. KOH examination of nail scrapings. This is because the patient's symptoms and physical examination findings are suggestive of a fungal infection of the nail, also known as onychomycosis. KOH examination of nail scrapings can confirm the presence of fungal elements in the nail, guiding the further management and treatment options.