Final answer:
Iron supplements are commonly associated with causing constipation, making them the most likely cause among the listed options when a client reports constipation after increasing mineral supplement intake.
Step-by-step explanation:
The client who reports constipation during a routine checkup and has increased their intake of mineral supplements might be experiencing constipation due to iron supplementation. Iron is a mineral known to cause gastrointestinal side effects such as constipation, especially when taken in high doses or on an empty stomach. While other minerals like magnesium may actually help alleviate constipation, and calcium can contribute to constipation when consumed in excess without adequate magnesium, iron is the most likely cause among the options given. It's important to balance iron intake with adequate fluid and fiber in the diet and to consult with a healthcare provider regarding appropriate dosage and potential need for stool softeners.