Final answer:
A follicular cast is caused by an excessive accumulation of keratin in the hair follicle, leading to plugs that adhere to it.
Step-by-step explanation:
The cause of a follicular cast is C. Accumulation of keratin in the hair follicle. Follicular casts occur when there is an abnormal amount of keratin that has built up and adhered to the hair follicle, often appearing as cylindrical plugs. While factors like genetic predisposition and hormonal imbalances can influence skin conditions, excess keratin accumulation is directly responsible for follicular casts. The hair matrix mentioned in some of the reference information contains a layer of basal cells, not the follicle itself, and it is the source of new hair. Conditions affecting the hair matrix, follicles, or surrounding structures can lead to hair and skin abnormalities, including the formation of follicular casts.