Our cats have developed yellow oily, waxy patches on their backs at the base of their tails. The veterinarian called it "stud tail." What could be causing this?

Oil from the sebaceous glands.

Cats and dogs have an area at the base of their tails that is dense with sebaceous glands that produce a waxy or oily yellowish secretion. When these secretions accumulate the hair may become matted and the skin may crust and scale. The area may become secondarily infected with bacteria in rare cases. The condition is termed feline tail gland hyperplasia, but is commonly referred to as "stud tail" because the condition is prevalent in uncastrated males.

The condition is not highly pathologic, and usually is more of a cosmetic nuisance to owners and cats. The fur in the involved area should be clipped and washed with medicated antiseborrheic shampoo, which may contain benzoyl peroxide or sulfur-salicylic acid; your veterinarian can provide you with these shampoos. Hopefully the cats will resume grooming the cleaned area and that should prevent recurrence. If your cats do not keep the area well groomed, then you should gently use a comb to treat the fur and to disperse the oils that accumulate. Systemic medications are rarely needed to deal with the problem.