Final answer:
The Hypothalamus, which is in charge of body temperature regulation, was most likely affected when Gustav collapsed after drinking a new sports drink.
Step-by-step explanation:
The part of the brain most likely affected if Gustav collapsed with a high temperature after consuming large amounts of a new sports drink is the Hypothalamus. The hypothalamus is responsible for regulating the body's temperature, and if Gustav's temperature was high enough to necessitate a rush to the emergency room, it suggests that his hypothalamus may have been compromised. Factors such as consuming large quantities of substances containing electrolytes or other compounds could disrupt the hypothalamus's ability to maintain homeostasis. Note: Since the exact contents of the sports drink are not known, other areas of the brain or systemic effects could also be implicated, but based on the information given, the hypothalamus is the most likely candidate.