Question

Which of the following pairs is the least variable constituent in milk during the stage of lactation?

• A. Lactose & chloride
• B. Lactose & protein
• C. Fat & protein
• D. Protein & chloride

Hint:

Milk Constituent Variability:
Least Variable (Strict Osmotic Balance) = LACTOSE & CHLORIDE.
Most Variable = FAT (Highly variable by breed, season, feed, milking stage).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Osmotic homeostasis of milk: lactose ($pprox 50\%$) and soluble chloride/potassium/sodium electrolytes maintain constant osmotic equilibrium with blood plasma ($300 ext{ mOsm/kg}$), making the lactose-chloride balance the least variable thermodynamic parameter throughout lactation.
Key Formula or Approach:
\[ \text{Osmotic Pressure of Milk} = \text{Osmotic Pressure of Blood} \approx 300\text{ mOsm/kg} \iff \mathbf{\text{Lactose} \uparrow \longleftrightarrow \text{Chloride} \downarrow} \]

Step 2: Detailed Explanation:
In milk secretion physiology and compositional stability:
- Secretion of water into milk in the mammary alveolar lumen is driven strictly by the osmotic gradient created by Lactose synthesis.
- Milk is maintained strictly isosmotic with maternal blood plasma (freezing point depression constant at $-0.530^\circ ext{C}$ to $-0.550^\circ ext{C}$).
- If lactose concentration fluctuates slightly, mammary epithelial ion channels immediately adjust Chloride ($\text{Cl}^-$) and sodium influx in a strict reciprocal inverse relationship (Koestler Number: $[\%\text{ Chloride} \times 100] / \%\text{ Lactose}$).
- Together, the Lactose & Chloride pair (A) constitutes the least variable and most tightly physiologically regulated constituent pair in normal milk to preserve osmotic homeostasis throughout lactation. (In contrast, Milk Fat and Protein exhibit major cyclic variability).

Step 3: Final Answer:
Therefore, the least variable pair is Lactose & chloride, corresponding to option (A).