Review Question - QID 213231

A 39-year-old woman presents to the clinic with complaints of constipation for the past 2 weeks. She reports that it has been getting increasingly difficult to pass stool to the point that she would go for 2-3 days without going to the bathroom. Prior to this, she passed stool every day without difficulty. She denies weight changes, headaches, chest pain, or abdominal pain but endorses fatigue. Her past medical history is significant for 2 episodes of kidney stones within the past 3 months. A physical examination is unremarkable. Laboratory studies are done and the results are shown below:

Serum:
Na+: 138 mEq/L
Cl-: 97 mEq/L
K+: 3.9 mEq/L
HCO3-: 24 mEq/L 
BUN: 10 mg/dL
Glucose: 103 mg/dL
Creatinine: 1.1 mg/dL
Thyroid-stimulating hormone: 3.1 uU/mL
Ca2+: 12.1 mg/dL
Phosphate: 1.2 mg/dL (Normal: 2.5-4.5 mg/dL)

What is the most likely explanation for this patient’s low phosphate levels?