Certified Surgical First Assistant (CSFA) Practice Test

The choice of 0.9% normal saline (NS) as the isotonic solution closest to human physiology is based on its osmolarity and ion content, which closely mimic that of plasma. Normal saline contains sodium and chloride ions in concentrations that align with physiological levels, making it an effective option for fluid resuscitation and maintenance in various clinical scenarios. This isotonic solution maintains extracellular fluid volume and helps restore electrolyte balance without causing significant shifts in fluid across cellular membranes.

In contrast, 0.45% NS is considered hypotonic and may lead to cellular swelling, while 5% dextrose is a solution that, once metabolized, becomes hypotonic as it provides primarily free water. Ringer's Lactate, while useful for certain clinical conditions, includes other components like lactate, which the body metabolizes, introducing additional variables that may not correlate perfectly with physiological saline concentrations. Thus, 0.9% normal saline stands out as the most balanced choice that directly reflects human plasma conditions in terms of osmolarity and composition.