**Core Concept**
Microsomia refers to a condition where an infant's birth weight falls below a certain percentile, indicating that they are smaller than a significant portion of the general population. This can be a concern for the infant's health and development. The percentile used to define microsomia is critical in determining the severity of the condition.

**Why the Correct Answer is Right**
The correct answer is Bih weight below 10th percentile because this threshold is commonly used in pediatric medicine to define microsomia. Infants with birth weights below the 10th percentile are considered small for their gestational age (SGA) and are at a higher risk of various health complications, including respiratory distress, hypoglycemia, and developmental delays. This is because growth restriction during fetal development can lead to inadequate nutrient and oxygen supply, resulting in reduced fetal growth.

**Why Each Wrong Option is Incorrect**
**Option A:** Bih weight below 90th percentile is not a definition of microsomia, as this would actually indicate an above-average birth weight.

**Option C:** Bih weight below 20th percentile is also not a definition of microsomia, as this would only indicate that the infant is slightly smaller than average.

**Option D:** Bih weight below 50th percentile is not a definition of microsomia, as this would indicate that the infant is only slightly smaller than average and may not require special medical attention.

**Clinical Pearl / High-Yield Fact**
It is essential for pediatricians to accurately assess birth weight and gestational age to identify infants at risk of microsomia and provide appropriate care and monitoring.

**✓ Correct Answer: B. Bih weight below 10th percentile**