**Core Concept**
Ponderal Index (PI) is a measure used to assess fetal growth and development. It is calculated as the birth weight in grams divided by the cube of the length in centimeters. This ratio helps to standardize growth references and identify small for gestational age (SGA) infants.

**Why the Correct Answer is Right**
A Ponderal Index of less than 2 is used to define a small for gestational age (SGA) infant. This is because a lower PI indicates that the infant's weight is disproportionately low compared to their length, suggesting intrauterine growth restriction. The World Health Organization (WHO) growth charts use a PI of less than 2 as the cutoff for SGA classification.

**Why Each Wrong Option is Incorrect**
**Option A:** A Ponderal Index of less than 1 would be more indicative of severe growth restriction, not just SGA.
**Option C:** A Ponderal Index of less than 3 is not a commonly used threshold for SGA classification.
**Option D:** A Ponderal Index of less than 4 would be within the normal range for most infants.

**Clinical Pearl / High-Yield Fact**
Remember that Ponderal Index is a useful tool for identifying SGA infants, but it should be used in conjunction with other growth parameters, such as birth weight and head circumference, for a comprehensive assessment.

**✓ Correct Answer: B. Less than 2**