challenge: What is the osmol gap in clinical laboratory testing?
  
  A. The sum of sodium, glucose, and blood urea nitrogen (BUN) levels in the blood
  B. The difference between the measured and calculated levels of sodium ions in the blood
  C. The difference between the measured and calculated levels of glucose in the blood
  D. The difference between the measured plasma osmolality and the calculated plasma osmolality
  Answer: 