Which medication may cause a single-lobed brain and neural tube defects during gestation?

The medication that is recognized for its potential to cause significant congenital malformations, including brain and neural tube defects during gestation, is isotretinoin. Isotretinoin is a retinoid used primarily for severe acne and is known to have teratogenic effects if taken during pregnancy.

The use of isotretinoin in pregnant women is contraindicated due to its association with a wide range of birth defects known collectively as "isoTretinoin embryopathy," which can include malformations such as hydrocephalus, microcephaly, and neural tube defects. These outcomes arise because isotretinoin affects the normal development of the fetal neural system during critical periods of embryogenesis.

While simvastatin, carbamazepine, and cyclophosphamide have their own potential risks and have been associated with various types of birth defects, isotretinoin is particularly notorious for its severe teratogenic effects specifically related to central nervous system development. As such, isotretinoin is considered especially high-risk during pregnancy, making it the most appropriate selection among the options given.