In many ways a mule is a superior animal to the horse or donkey. they are often stronger and can jump higher than either of their parents. why then are they considered to be a "weak hybrid"?
they are sterile.
their offspring are progressively weaker.
they do not live as long.
they are more prone to disease.

They are sterile.

Explanation:

A mule is the offspring of a male donkey (jack) and a female horse (mare). Thus the number of chromosome of the donkey and the horse are quite different.  A combination of the horse chromosome which is (64) and a that of the  donkey chromosome which is (62) gives rise to 63 chromosomes in a Mule.

This major difference in number,forms and morphology typically prevents the chromosomes from pairing properly and producing successful embryos, as a result of this aftermath effect, it renders majority of the mules to be sterile.

The correct answer is encephalocele.

Explanation:

The cystic hygroma is a malformation in the neck or upper abdominal area and holoprosencephaly is a malformation or alteration in the facial area, not in the cranial area.

The only one where the shape of the skull is altered (mainly the area of the calotte) is the encephalocele.