Vitamin A deficiency is most likely to explain this patient's findings, and the most appropriate and important study is immediate assessment of this patient's vitamin A status with serum retinol measurement. Her ocular symptoms are very serious manifestations of vitamin A deficiency, which may be progressive and may lead to permanent visual impairment if left untreated. Symptoms associated with vitamin A deficiency include decreased vision at night or in dim light, dry eyes, corneal and/or eyelid inflammation, and rough and/or dry skin. The absorption of the fat-soluble vitamins A, D, E, and K can be severely impaired following malabsorptive bariatric surgery, including Roux-en-Y gastric bypass (RYGB) and biliopancreatic diversion with duodenal switch. Vitamin A deficiency has been reported to occur in 11% of patients after gastric bypass despite taking a daily multivitamin. This deficiency can occur within 1 year of surgery; the reported prevalence is 10% to 50% in patients with RYGB and 61% to 69% in those with biliopancreatic diversion. Vitamin A deficiency is believed to arise from multiple factors such as fat malabsorption, decreased intake from reduced overall food consumption, and possible underlying fatty liver disease. Other micronutrient deficiencies (in addition to the previously mentioned fat-soluble vitamins A, D, E, and K) that can develop following RYGB include iron, folic acid, zinc, selenium, copper, magnesium, thiamine (vitamin B₁), cobalamin (vitamin B₁₂), vitamin C, and in rare cases, riboflavin (vitamin B₂) and pyridoxine (vitamin B₆).

Copper deficiency may cause muscle weakness due to myeloneuropathy, ataxia, and cognitive deficits that may be difficult to differentiate from B₁₂ deficiency. Less common neurologic findings include bilateral visual loss. Hematologic findings in copper deficiency include microcytic anemia, which may mimic iron deficiency anemia, and leukopenia.

Although iron deficiency does occur following RYGB, this patient is on iron replacement and has a normal mean corpuscular volume. Patients with iron deficiency are likely to have hypochromic, microcytic anemia and may have brittle or deformed nails, cheilitis, pica, and restless legs syndrome. Visual symptoms are not seen with iron deficiency anemia.

Although vitamin B₁₂ deficiency may occur following RYGB, this is unlikely given this patient's normal mean corpuscular volume. Furthermore, a vitamin B₁₂ deficiency could lead to peripheral neuropathy and gait disturbance but not the skin or visual disturbances seen in this patient.