A yellow leaf can be useful evidence, but only if its position and pattern are recorded. Photograph the whole plant first, then the affected leaf and nearby healthy growth in similar light.
Note whether the leaf is old or newly formed, where it sits on the plant and whether the veins look different from the tissue around them. A detached leaf on a table loses much of that context.
Add the growing history
Record recent watering, feeding, repotting and moves. Check for pests and look at the compost. Nutrient shortage is one possibility, but a photograph alone cannot confirm it.
Do not increase several nutrients at once to match several possible diagnoses. That can obscure the original pattern and complicate the next step.
Keep the images dated. If symptoms move to new leaves or spread around the plant, the sequence is more informative than a single close-up. When seeking help, include the botanical name if known, the potting medium and the root conditions. Saying that a plant has 'yellow leaves' is the beginning of the enquiry, not its conclusion.
Related reading: Yellow Leaves and Nutrient Deficiency: Patterns to Check.