A 52-year-old woman presents with a 2 cm ulcerated lesion on the vulva. Biopsy confirms squamous cell carcinoma. On examination, she has palpable lymphadenopathy in the groin. The structure marked **A** in the diagram represents the first group of lymph nodes likely to be involved in the metastatic spread of her vulvar cancer. Which of the following best describes the lymphatic drainage pattern that makes this group the sentinel node site for vulvar malignancy?