Which crystalloid has osmolality nearest to serum osmolality-
**Core Concept**
The osmolality of a crystalloid solution is a critical factor in its choice for fluid resuscitation, as it affects the distribution of water between the intravascular and extravascular spaces. Crystalloids with osmolality close to that of serum (approximately 285-295 mOsm/kg) are ideal for maintaining intravascular volume and preventing fluid shifts.
**Why the Correct Answer is Right**
Normal saline (0.9% NaCl) has an osmolality of approximately 308 mOsm/kg, which is close to the osmolality of human serum (285-295 mOsm/kg). This makes normal saline an excellent choice for fluid resuscitation, as it does not cause significant fluid shifts or changes in serum osmolality. The close osmolality of normal saline to serum osmolality also helps to maintain the balance of electrolytes and prevent complications such as hyperchloremic acidosis.
**Why Each Wrong Option is Incorrect**
**Option A:** Ringer's lactate has an osmolality of approximately 273 mOsm/kg, which is lower than serum osmolality. While it is a commonly used crystalloid, its lower osmolality can lead to fluid shifts and changes in serum osmolality.
**Option B:** D5W (5% dextrose in water) has an osmolality of approximately 252 mOsm/kg, which is significantly lower than serum osmolality. This can lead to hypotonicity and fluid shifts, making it a poor choice for fluid resuscitation.
**Option C:** Hypertonic saline has an osmolality of approximately 1020 mOsm/kg, which is much higher than serum osmolality. This can lead to significant fluid shifts and complications such as central pontine myelinolysis.
**Clinical Pearl / High-Yield Fact**
Normal saline is the preferred choice for fluid resuscitation in most clinical scenarios, as it maintains intravascular volume and prevents fluid shifts. However, it can lead to hyperchloremic acidosis in patients with pre-existing renal impairment or those receiving large volumes.
**Correct Answer:** C. Hypertonic saline