**Question:** Total body water differences between male and female is not seen at the age of

A. Infancy
B. Puberty
C. Adolescence
D. Old age

**Core Concept:** Total body water (TW) plays a crucial role in maintaining body homeostasis and consists of extracellular and intracellular water. TW is influenced by sex hormones, particularly estrogen and progesterone.

**Why the Correct Answer is Right:** The correct answer, **Infancy**, is based on the understanding that sex differences in total body water are primarily determined by sex hormones. In infants, the levels of sex hormones are very low, leading to minimal differences in TW between males and females. As the child grows, sex hormones start to influence TW levels, which is why we see sex differences in TW during puberty (Option B), adolescence (Option C), and old age (Option D).

**Why Each Wrong Option is Incorrect:**

Option A (Infancy): As explained above, sex hormone levels are very low during infancy and do not contribute to the observed sex differences in TW.

Option B (Puberty): The sex hormone levels are still increasing during puberty, which is why we do see sex differences in TW during this stage. The difference between males and females is more pronounced during puberty due to the influence of sex hormones on TW.

Option C (Adolescence): Although sex hormone levels continue to increase during adolescence, the difference in TW between males and females is less distinct than during puberty. This is because sex hormones have already started to influence TW in infancy and childhood, leading to a more gradual sex difference in TW.

Option D (Old Age): In old age, sex hormones levels decrease, leading to less distinct sex differences in TW. This is opposed to the situation in infancy and childhood, where sex hormones have a more significant impact on TW.

**Clinical Pearls:**

- Understanding the influence of sex hormones on total body water helps in explaining the observed differences between males and females at various stages of life.
- Sex hormones play a significant role in determining sex differences in total body water, which is important for healthcare professionals to know when assessing and managing patients of different genders and ages.

**Correct Answer: Infancy**

**Explanation:** Infancy is the stage where sex hormones have minimal influence on total body water, resulting in minimal sex differences in total body water between males and females at this stage.