Identify a key indicator of substance use disorder.

Increased tolerance is a significant indicator of substance use disorder because it reflects the body's adaptive response to prolonged exposure to a substance. When an individual develops tolerance, they require larger amounts of the substance to achieve the same effect that was once produced by a smaller amount. This escalation often points to a physical dependence on the substance, as the body has adjusted its chemistry to accommodate the presence of the drug. This process can lead to a cycle of increased consumption and potential escalation of other problematic behaviors associated with substance use, culminating in a disorder.

The other choices, although they may be relevant in the context of mental health or behavioral changes, do not specifically indicate substance use disorder. Frequent mood changes can occur due to various factors, including stress or mental health issues, and are not unique to substance use. The introduction of animals in therapy pertains to therapeutic techniques rather than indicators of substance use. Social withdrawal is a common symptom in many mental health conditions as well and does not directly indicate substance use disorder on its own. Therefore, increased tolerance stands out as a key indicator due to its direct relationship with the physiological impact of substance use.