Final answer:
The question does not provide enough information to accurately answer it.
Step-by-step explanation:
The question seems to be referring to a specific incident or story where the youngest brother buys poison. However, the information provided does not give any specific context or details about the incident. Without more information, it is not possible to accurately answer the question. The question "Why did the youngest brother buy poison?" is unclear without a specific context or literary reference. In history, particularly in the story of Ariq Böke, the youngest brother might buy or acquire poison if he was involved in political rivalries that often led to assassination attempts, such as the power struggles that occurred during the Mongol empire succession debates. Poison was a common tool for political assassinations throughout history. In literature, the motif of the youngest brother is frequently found, and the reasons for buying poison can be diverse, ranging from jealousy, such as in the Biblical story of Cain and Abel, to desperation or revenge, as seen in various ancient myths and stories. In the context of the query, it is essential to have more information to accurately determine the reason. Nonetheless, in both history and literature, the purchase of poison by a younger brother character often leads to tragic outcomes and is typically driven by darker human emotions or dire situations.