Why is a Guru considered essential for removing ignorance, even for a wise person?

A Guru is indispensable because ignorance, or Ajnana, often manifests from doubt (vikalpa), which cannot be dispelled without a Guru's guidance. As Chapter 50 illustrates, when doubt enters knowledge, even a very wise person can become prideful and their knowledge corrupted. The text uses the analogy of a single drop of fermented gruel ruining a large pot of milk to show how doubt can spoil wisdom. Even a supreme knower like Shuka is said to have suffered a loss due to doubt. Therefore, as explained in Chapter 50, the removal of ignorance, which purifies the mind, requires the help of a Guru.