Assignment task:
Mr. B, a 40-year-old avid long-distance runner previously in good health, presented to his primary provider for a yearly physical examination, during which a suspicious-looking mole was noticed on the back of his left arm, just proximal to the elbow. He reported that he has had that mole for several years, but thinks that it may have gotten larger over the past two years. Mr. B reported that he has noticed itchiness in the area of this mole over the past few weeks. He had multiple other moles on his back, arms, and legs, none of which looked suspicious. Upon further questioning, Mr. B reported that his aunt died in her late forties of skin cancer, but he knew no other details about her illness. The patient is a computer programmer who spends most of the work week indoors. On weekends, however, he typically goes for a 5-mile run and spends much of his afternoons gardening. He has a light complexion, blonde hair, and reports that he sunburns easily but uses protective sunscreen only sporadically.
Physical exam revealed: Head, neck, thorax, and abdominal exams were normal, with the exception of a hard, enlarged, non-tender mass felt in the left axillary region. In addition, a 1.6 x 2.8 cm mole was noted on the dorsal upper left arm. The lesion had an appearance suggestive of a melanoma. It was surgically excised with 3 mm margins using a local anesthetic and sent to the pathology laboratory for histologic analysis. The biopsy came back melanoma.
Question: What is the pathophysiology of melanoma as well as the enlarged lymph node found on exam?