The sphere of the prefixes shown above is vast and may be confusing. These "addenda" to words populate medical, legal and other professional parlance as well as classic literature. The words shown above are illustrative. As usual, we include artistic depictions to add some levity to the subject.

The prefix A as in AN-encephaly denotes a "void" more than an AB-normality. In this sense it lacks the implication of "away from normal" but rather a more neutral posture if "not there". The example of AN-encephaly implies an AN-omaly. It is not an A-genesis of the brain because we know "it was there" but it died and vanished. Therefore, it appears that Anencephaly is not an A-genesis or A-plasia (lack of gene translation into cells and tissues) but a DYS-genesis or an AB-normality. However, there are, of course, instances when an Anencephaly could result from an A-genesis, implying lack of essential genes as in the case of some chromosomal AB-normalities. Note that anencephaly is a common human lethal congenital malformation. Use of folic acid supplements reduces the frequency of such anomalies of the neural tube malformations by about 50%. This does NOT mean that lack of folates is a cause of such malformations of the neural tube. Furthermore, the causes of NTD can be multiple as suggested by the 50% of instances that persist even of supplements of folic acid are used before and during early pregnancy. When the PLASICITY of tissues is abnormal, the term DYS-plasia comes to play. AN-encephaly is an example of DYS-morphology or abnormal morpho-genesis (generation of shapes). DYS or DIS-ease are two prefixes opposite to EU as in EUgenics that stand for what is "optimal". That which is not, may usher DYS or DIS as in dysplasia or disease as well as other DIS-ruptions. We have now approached the gates of the sphere of AB-normality using an example of MAL-formation which in vulgar and politicized parlance may be called "Birth Defect".
 
The prefix AB then, in contrast to A which implies "lack of", stands for "removal" or "negation" of "norm" or "normal". We know that those in the majority think of themselves as "the NORM". Proud humans easily expand their "self-delusions" and claim to represent the "norm" while branding others as "different" or "defective" - "birth defect" can serve as an example. Most persons have one or another "defect" which have zero medical implications or put in another way, "nobody is perfect".
 
Another panorama of AB  is illustrated by AB-duct, AB-domen and so on. Note that the concept of "NORM" can imply "MEAN" or average which implicitly connects with MIDDLE. Here the logic gets strained - we ABduct property, children and other goods in the sense that we move "away" things from their sites. ABDUCTOR muscles are those who "move away from the "mean" or middle. Moving a thigh away from the mid-line requires the action of ab-ductor musculature. The opposite is the command "ADDUCT" your thighs which implies approximate your knees toward the middle. In essence, "ADDUCT" means "toward" and "ABDUCT" means "away" but ... we have two arms, two legs, two eyes, thus the body has a "middle" and so do each of these structures ... the rule is, the terminology relies on two factors; the "tip end" of the structure we refer to and weather the tip points "out" or "in". For example, a bunion or deviation of the hallux or great toe illustrated below. In this instance adduct and abduct are renamed as VARUS and VALGUS. To repeat, "tip away" is VALGUS" and "tip inward" is VARUS" (plurals are VALGA and VARA).  Below is illustrated and example of HALLUX (big toe) VALGUS (tip away from the middle of the body - not of the foot).

The VALGUS - VARUM terms are often applied to describe our knees. The position of our knees with respect to our feet (in this instance often are "away from the norm". Given that the knee-foot are the reference points, anomalies can be represented schematically by a pairs of parentheses, as follows:  () and () respectively. If the starting point are the knees (GENU) and the "point away" are the feet, GENU VALGUS implies "feet away" as in the scheme )( often called "knowk knees". The contrary is GENU VARUM or "feet inward" as in the scheme () often called "bowed legs". Below is an example of "bowed arms" in a non-human quadrui-ped ("cuatro" in Spanish is four and "ped" from "podos" in Greek is "oot").