B  
Isotretinoin (a retinoic-acid derivative) is a potent human teratogen that produces a characteristic embryopathy with craniofacial anomalies (including cleft palate and microcephaly), central-nervous-system defects such as neural-tube malformations, and congenital heart defects such as atrial or conotruncal septal abnormalities [Lammer, 1985, PMID 3162101].