The correct answer is Alcohol.

- Fetal Alcohol Syndrome (FAS) is a condition resulting from prenatal exposure to alcohol.
- When a pregnant woman consumes alcohol, it crosses the placenta and interferes with normal fetal development, especially affecting the brain and craniofacial structures.
- The syndrome is characterized by a range of clinical features, including microcephaly (a smaller than normal head size), distinctive facial abnormalities, growth retardation, and most importantly, cognitive impairments such as learning disabilities, behavioral problems, and intellectual disability.

- While other recreational drugs like cannabis, cocaine, and methamphetamine also pose risks during pregnancy, none are specifically linked to the classic syndrome characterized by microcephaly and the particular pattern of cognitive impairments seen in FAS. Alcohol remains the most well-documented and common cause.

Key points:
- Alcohol consumption during pregnancy is teratogenic.
- Causes fetal alcohol syndrome, characterized by microcephaly, facial dysmorphisms, growth deficiency, and neurocognitive impairment.
- Diagnosis relies on clinical features and history of prenatal alcohol exposure.
- Prevention involves abstinence from alcohol during pregnancy.

Reference: Robbins Basic Pathology, 10th Edition, Chapter 15, Page 624