What is a vital sign that should be regularly monitored in an animal during a wellness check?

Monitoring body weight is crucial during a wellness check for several reasons. Body weight is a primary indicator of an animal's overall health and well-being. Fluctuations in weight can signify various health issues, such as nutritional deficiencies, metabolic disorders, or underlying medical conditions that may require further investigation or intervention. Regularly tracking an animal's weight helps in assessing their diet, growth (in young animals), and can also indicate if there are any changes in their activity level or health status over time.

The other options, while they may have some relevance in specific contexts, do not provide the comprehensive insight into an animal's health that body weight does. For example, earwax color is not routinely monitored and can vary widely without being indicative of health problems. Nail length is typically a maintenance concern rather than a direct indicator of health status. The favorite toy of an animal, while it might contribute to its happiness, does not inform on the physical health or vital signs that are essential for well-being. Thus, body weight stands out as the most vital sign to monitor during a wellness check.