Identify one key symptom of a nail infection.

Discoloration or foul odor serves as a key symptom of a nail infection because these indicators often reflect a biochemical change due to an infection, typically caused by bacteria or fungi. When a nail becomes infected, it can change color (such as becoming yellow, green, or dark) as a reaction to the pathogens present. A foul odor may also develop due to the accumulation of pus or necrotic tissue as a result of the infection.

The other symptoms mentioned may not directly indicate an infection. For instance, dryness of the nail can be related to various factors, including environmental conditions or damage but does not specifically suggest an infection. Similarly, pain during filing could be associated with other nail conditions or injuries and is not a definitive symptom of infection. Excessive shine can indicate healthy, well-moisturized nails but does not correlate with an infection scenario. Thus, discoloration or foul odor is a strong sign that points to the presence of a nail infection.