**Core Concept**
Failure to thrive (FTT) in infants and children is a condition characterized by inadequate nutrition, leading to poor physical growth and development. It is a concern in pediatric medicine as it can result from various underlying causes, including inadequate caloric intake, malabsorption, or increased energy expenditure.

**Why the Correct Answer is Right**
Inadequate nutrition leads to poor weight gain and stunted growth in children. The World Health Organization (WHO) growth charts are used to assess growth and development in children. Failure to thrive is typically diagnosed when a child's weight for age is below the 5th percentile, indicating that the child is significantly underweight compared to their peers. This cutoff is chosen because it allows for a margin of error in measurement and takes into account the natural variability in growth rates among children.

**Why Each Wrong Option is Incorrect**
* **Option A:** The 10th percentile is too high a cutoff for diagnosing failure to thrive, as it may not accurately capture children who are at risk of poor growth and development.
* **Option B:** The 25th percentile is also too high, as children who fall below this cutoff may still be at risk of growth faltering, but may not require intervention.
* **Option C:** The 5th percentile is the correct answer, but the other options are not provided.

**Clinical Pearl / High-Yield Fact**
A useful mnemonic to remember the definition of failure to thrive is: "FTT = F (Food) + T (Time) + T (Threshold)". This reminds clinicians that failure to thrive is a condition characterized by inadequate nutrition (F), which occurs over a prolonged period of time (T), and is typically diagnosed when a child's growth falls below a certain threshold (T).

**Correct Answer:** **5**. The child's weight for age is below the 5th percentile.