What is one negative effect of untreated pain in animals?

Untreated pain in animals commonly leads to weight loss and increased susceptibility to infection, making this answer the most accurate choice. Pain can cause an animal to exhibit a decrease in appetite, which contributes to weight loss as they are less likely to eat properly. Furthermore, chronic pain can result in stress and a weakened immune response, making the animal more vulnerable to infections.

The other choices do not accurately align with the physiological and behavioral consequences of untreated pain. Increased energy and activity levels would be contrary to the typical response of an animal in pain, which usually becomes more lethargic and withdrawn. Improved immune system function is unlikely, as pain and stress are more associated with a compromised immune response. Additionally, while pain can affect learning and memory, it is generally in a negative way, leading to decreased cognitive function rather than enhancement.