Final answer:
A patient with well-circumscribed brown, oval, rough papules with a "stuck-on" appearance likely has benign Seborrheic keratosis, a common non-cancerous skin growth.
Step-by-step explanation:
If a patient suddenly has an eruption of many well-circumscribed brown, oval, rough papules with a "stuck-on" appearance on his trunk and proximal extremities, this is most likely a benign finding. The correct diagnosis would be A) Seborrheic keratosis. This is a common skin growth seen in older adults, which can appear anywhere on the body, especially on the trunk and head. Seborrheic keratosis has a characteristic "stuck-on" appearance and is not cancerous. In contrast, basal and squamous cell carcinomas, which are forms of skin cancer, have different features and are less likely to have a "stuck-on" look. They are often more irregular, can be scaly or pearly, and unlike benign lesions like seborrheic keratosis, they can pose a risk due to their potential to grow and invade surrounding tissues.