## Core Concept
Total body water (TBW) refers to the total amount of water in the human body, which is crucial for various physiological functions. The percentage of TBW in the human body varies by age, sex, and body composition. In infants and young children, TBW is relatively higher compared to adults.

## Why the Correct Answer is Right
A one-year-old child has a higher percentage of TBW compared to adults due to their higher proportion of body water relative to their body weight. At birth, TBW is approximately 70-80% of body weight, and it decreases as the child grows. By one year of age, TBW is about 60% of body weight. This is because infants have less body fat and a higher proportion of lean body mass, which contains more water.

## Why Each Wrong Option is Incorrect
* **Option A:** 40% - This is too low for a one-year-old child, as it is more representative of elderly adults or individuals with significant dehydration.
* **Option B:** 50% - While getting closer, this still underestimates the TBW percentage for a one-year-old child.
* **Option D:** 70% - This overestimates the TBW for a one-year-old child, as it is more representative of newborns.

## Clinical Pearl / High-Yield Fact
A key point to remember is that TBW decreases as a person ages, from about 70-80% at birth to approximately 55-60% in adults. For pediatric patients, especially in the context of fluid management, knowing that a one-year-old child has approximately 60% TBW is crucial.

**Correct Answer:** . 60%