If abnormal development occurs in the myelencephalon, which of the following structures may be affected?

A) Fourth ventricle
B) Medulla oblongata
C) Thalamus
D) Both "Fourth ventricle" and "Medulla oblongata" are correct.

Myelencephalon

The myelencephalon is the most posterior region of the brainstem, comprising the medulla oblongata and responsible for controlling vital autonomic functions such as respiration and heart rate.

Fourth Ventricle

A chamber of the brain located between the brainstem and the cerebellum, through which cerebrospinal fluid circulates.

Medulla Oblongata

A portion of the brainstem that regulates vital bodily functions, such as heart rate, breathing, and blood pressure.