Final answer:
Spina bifida is often diagnosed with a prenatal ultrasound and can be detected as early as 21 weeks gestation. It is a birth defect where the neural tube does not close completely, leading to protrusion of meninges and neural tissue. Other conditions like spondylolisthesis, scoliosis, and ankylosing spondylolysis are not commonly diagnosed prenatally.
Step-by-step explanation:
Among the conditions listed, spina bifida is often diagnosed with a prenatal ultrasound. Spina bifida is a birth defect of the spinal cord that occurs when the neural tube does not completely close, resulting in meninges and neural tissue protruding through the vertebral column. This can be detected on an ultrasound, as seen in fetal myelomeningocele at 21 weeks gestation.
There are three classes of spina bifida: occulta, meningocele, and myelomeningocele. Spina bifida occulta is the mildest form and may not present functional differences. Meningocele involves the meninges protruding through the spinal column without necessarily involving nerves. Myelomeningocele, the most severe form, involves both meninges and spinal nerves protruding, causing severe neurological symptoms.
The other conditions listed, such as spondylolisthesis, scoliosis, and ankylosing spondylolysis, are spinal disorders associated with the abnormal curvature or alignment of the spine, and they are generally not diagnosed prenatally through an ultrasound.