Question

A 45-year-old woman presents to the clinic with a 3-month history of chronic, watery diarrhea. She reports having 4 to 6 large-volume, loose bowel movements per day and notes that the diarrhea significantly improves when she fasts or avoids meals. Her medical history is notable for type 2 diabetes mellitus, which is managed with metformin and a recently initiated alpha-glucosidase inhibitor (acarbose). Her vital signs are within normal limits, and physical examination is unremarkable.

Laboratory results are as follows:

Serum chemistry:
- Sodium: $140$ mEq/L
- Potassium: $3.8$ mEq/L
- Chloride: $102$ mEq/L
- Bicarbonate: $24$ mEq/L

Stool studies:
- Stool osmolality: $290$ mOsm/kg
- Stool sodium: $40$ mEq/L
- Stool potassium: $35$ mEq/L
- Fecal leukocytes: Negative
- Fecal occult blood: Negative

Based on the provided laboratory values, calculate the patient's stool osmotic gap in mOsm/kg.