Can you explain the relationship between doubt, ignorance, and the role of a Guru as described in the source?

The text from Chapter 50 establishes a critical relationship between doubt, ignorance, and the necessity of a Guru. It states that ignorance arises from doubt ('vikalpa') and emphatically asserts that this ignorance "cannot be removed without a Guru." To illustrate the danger of doubt, it mentions that even a supreme knower like Shuka suffered a loss because of it. When doubt enters knowledge, it is likened to a drop of fermented gruel that ruins a large pot of milk, making even a wise person prideful. Therefore, the Guru's role is indispensable for removing the doubt that fosters ignorance, thereby purifying the mind so that true knowledge can manifest.