Normal plasma osmolality is
**Core Concept**
Normal plasma osmolality is a critical parameter that reflects the balance between the concentration of solutes in the plasma and the water content in the body. It is essential for maintaining proper cellular function and preventing dehydration or overhydration. The normal range of osmolality is around 285-295 mOsm/kg.
**Why the Correct Answer is Right**
The correct answer is a value within the normal range of plasma osmolality. This range is maintained by the balance between the concentration of sodium, glucose, and urea in the plasma. The kidneys play a crucial role in regulating osmolality by adjusting the amount of water and solutes in the urine. The hypothalamus also regulates osmolality by releasing antidiuretic hormone (ADH) to increase water reabsorption in the kidneys or by releasing vasopressin to increase water reabsorption in the collecting ducts.
**Why Each Wrong Option is Incorrect**
**Option A:** This value is too low, indicating hyponatremia or overhydration, which can lead to cellular swelling and potentially life-threatening complications.
**Option B:** This value is too high, indicating hypernatremia or dehydration, which can cause cellular shrinkage and potentially lead to seizures or coma.
**Option C:** This value is outside the normal range and may indicate a condition such as hyperosmolar hyperglycemic nonketotic syndrome (HHNS) or diabetic ketoacidosis (DKA), which requires immediate medical attention.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that a plasma osmolality of 290 mOsm/kg is often used as a reference value, and values outside this range may indicate a need for further investigation and treatment.
**Correct Answer: C. 285-295 mOsm/kg**