Question

A child is born with cystic fibrosis, an inherited recessive disorder.
No one in the family (on either side) for several generations is known to carry or have the disorder. How can this be explained?
The child contracted the illness from another child when he was born.
The child developed the disorder when he was born; it is a result of a birth defect.
The genetic disorder formed from the mother's poor nutrition and prenatal care.
The genetic disorder is recessive, and can be carried by heterozygous individuals without ever being realized or identified.