**Core Concept**
The screening test for neonatal hypothyroidism is based on the measurement of thyroid-stimulating hormone (TSH) levels in newborns. This test is crucial for early detection and treatment of congenital hypothyroidism, which can lead to intellectual disability and growth retardation if left untreated.

**Why the Correct Answer is Right**
The correct screening test is the TSH-based assay, which measures the level of TSH in the blood. Elevated TSH levels indicate primary hypothyroidism, whereas suppressed TSH levels suggest hyperthyroidism. The TSH-based assay is more specific than the older method of measuring thyroxine (T4) levels alone, as it can differentiate between central and peripheral causes of hypothyroidism. The TSH-based assay typically involves a heel prick blood sample, which is collected from the newborn's heel within the first few days of life.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as there is no established screening test for neonatal hypothyroidism that involves measuring free thyroxine (FT4) levels directly. While FT4 levels can be used to diagnose hypothyroidism, they are not used for neonatal screening.

**Option B:** This option is incorrect as the T4-based assay is an older method that has been largely replaced by the TSH-based assay. The T4-based assay can fail to detect central hypothyroidism, where TSH levels are suppressed despite low T4 levels.

**Option C:** This option is incorrect as the T3-based assay is not commonly used for screening neonatal hypothyroidism. T3 levels can fluctuate widely in newborns, making it a less reliable marker for congenital hypothyroidism.

**Option D:** This option is incorrect as there is no established screening test for neonatal hypothyroidism that involves measuring thyroid peroxidase antibodies (TPOAb). TPOAb levels can be used to diagnose autoimmune thyroiditis, but they are not used for neonatal screening.

**Clinical Pearl / High-Yield Fact**
The TSH-based assay is the recommended screening test for neonatal hypothyroidism due to its high sensitivity and specificity. It is essential to remember that the TSH-based assay should be performed within the first few days of life, as delayed testing can lead to missed cases of congenital hypothyroidism.

**Correct Answer:** B. T4-based assay