Final answer:
The patient's symptoms and clinical presentation suggest a branchial cleft cyst, which is congenital and characteristic when it becomes symptomatic due to infection, leading to surgical excision being planned.
Step-by-step explanation:
The underlying pathophysiology of the patient's illness with a chief complaint of fever and neck pain with a neck mass lateral to the sternocleidomastoid muscle that has been present from birth and does not move with swallowing is likely a branchial cleft cyst. This diagnosis fits because branchial cleft cysts are congenital and often become symptomatic when infected, leading to warmth and tenderness. Surgical excision is a common treatment for branchial cleft cysts once they become problematic, as they can potentially lead to recurrent infections. The other options (A) Lymphadenopathy, (B) Thyroglossal duct cyst, and (D) Dermoid cyst have characteristics that do not match the clinical presentation described. For example, thyroglossal duct cysts typically move upwards with swallowing.