**Core Concept**
Intravascular volume depletion in infants is a critical condition that requires prompt recognition. The most sensitive indicator of this condition would be a parameter that reflects the body's compensatory mechanisms to maintain blood pressure and perfusion of vital organs. In infants, the cerebral and coronary circulations are highly sensitive to changes in blood volume.

**Why the Correct Answer is Right**
The correct answer is **A. Calf skin turgor**. The calf skin turgor test is a non-invasive method to assess the hydration status of an infant. When intravascular volume is depleted, the skin becomes less turgid due to decreased blood flow and reduced capillary refill. This is because the body prioritizes blood flow to vital organs, such as the brain and heart, at the expense of peripheral tissues like the skin. As a result, the calf skin turgor becomes less elastic and more easily depressed, indicating dehydration.

**Why Each Wrong Option is Incorrect**
* **Option B:** Capillary refill time is a useful indicator of peripheral perfusion, but it is not as sensitive as calf skin turgor in detecting early signs of intravascular volume depletion.
* **Option C:** Blood pressure is a critical parameter, but it may remain within normal limits until significant dehydration has occurred. Additionally, blood pressure can be influenced by various factors, making it less reliable as a sole indicator of intravascular volume depletion.

**Clinical Pearl / High-Yield Fact**
In infants, the most sensitive indicator of dehydration is often the behavioral and physical changes, such as decreased urine output, sunken fontanelle, and decreased skin turgor. These changes can precede any significant alterations in vital signs or laboratory values.

**Correct Answer: A. Calf skin turgor**