A patient presents with symptoms of muscle weakness, fatigue, and weight loss, and is found to have a deficiency in vitamin D. Which of the following is the most likely underlying cause of this deficiency?

Vitamin D deficiency can result from inadequate dietary intake, impaired absorption (e.g., due to celiac disease), or increased excretion (e.g., due to kidney disease). The patient's symptoms are consistent with a deficiency in vitamin D.