A 65-year-old woman with diabetes is diagnosed with rectal cancer. After receiving neoadjuvant chemoradiation, she undergoes a laparoscopic low anterior resection and 4 of 13 nodes were found to be positive for disease. She undergoes 8 cycles of treatment with FOLFOX (folinic acid, fluorouracil, and oxaliplatin). At her 6-month follow-up visit, she reports numbness, paresthesia, and occasional pain in her toes bilaterally. What is the most likely cause of her symptoms?