In the case of a patient with dyslipidemia, what lifestyle modification is likely to help improve his condition?

Quitting smoking is a significant lifestyle modification that can greatly improve dyslipidemia. Smoking has a detrimental effect on lipid levels and overall cardiovascular health. It is associated with lower levels of high-density lipoprotein (HDL) cholesterol, often referred to as "good" cholesterol. By quitting smoking, a patient can start to positively affect their lipid profile, leading to better cardiovascular outcomes.

In addition to improving HDL levels, quitting smoking also enhances endothelial function and reduces the risk of atherosclerosis, further contributing to a healthier cardiovascular system. This comprehensive positive impact makes quitting smoking a vital step in managing dyslipidemia and promoting overall heart health.

The other options, such as increasing carbohydrate intake, taking vitamin D supplements, or reducing vegetable intake, do not directly address or improve dyslipidemia or may even exacerbate the condition. Hence, quitting smoking stands out as the most effective lifestyle change in this scenario.