Which of the following could be a physical sign of substance abuse?

Bloodshot eyes serve as a physical sign of substance abuse due to the physiological effects of various substances on the body. Many drugs, including alcohol and marijuana, can cause the blood vessels in the eyes to expand or become inflamed, resulting in a reddened appearance. This change can be a direct indicator of recent use or intoxication.

Understanding that physical signs are often visible symptoms is vital in identifying substance abuse. While financial difficulties, sudden mood swings, and poor work performance may also relate to or result from substance abuse, they don't represent direct physical manifestations in the same way that bloodshot eyes do. Physical signs are specifically observable characteristics that can indicate a potential substance issue, making bloodshot eyes a clear example in this context.