The author's purpose for including words such as scarlet letter, glaringly, and threat is this:
b. They suggest the painful feelings associated with having scoliosis.
The purpose of the author's use of descriptive terms is to help the reader understand the way and manner that this sickness can turn out to be.
The threat and glaring nature of the sickness were to be painted in a vivid way and the only way to achieve this is by using words that communicate imagery. Often in writing, descriptive words such as this one are used to communicate the depth of a situation.
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