A 35-year-old woman presents with a 3-month history of chronic medial knee pain and a sensation of the knee 'giving way' during walking. On examination, she has a positive McMurray's test and a positive Thessaly test. Imaging reveals a vertical tear of the medial meniscus in the posterior horn. Which anatomical attachment of the medial meniscus is most likely disrupted, leading to the sensation of instability and the positive McMurray's test?