The most severe variant involves the presence of severely hypoplastic main pulmonary arteries and branch pulmonary arteries, alongside agenesis of the patent ductus arteriosus. Blood flow to the lungs comes from various dysplastic (malformed) blood vessels from the thoracic aorta called major aortapulmonary collateral arteries, these blood vessels narrow down as time goes on. Causes
Although this birth defect is congenital, the exact cause is unknown, and it may vary between children with the condition, the following factors have been known to influence the risk of a baby being born with the condition:

Genetics
The molecular genetics of this condition isnt known in most people with PA(VSD), however, there have been candidate genes found to be possibly implicated in the pathogenesis of this condition:
DNAH10
DST
FAT1
HMCN1
HNRNPC
TEP1
TYK2
NDRG4
TBX5
NKX2.5
GATA4There have also been copy number variants described in the medical literature as associated with PA(VSD):
Deletion in chromosome 16p11.2
Deletion in chromosome 5q35.3
Deletion in chromosome 5p13.1
Deletion in chromosome 22q11.2
Deletion in chromosome 15q11.2
Deletion in chromosome 8p23.2
Deletion in chromosome 17p13.2
Duplication in chromosome 5q14.1
Duplication in chromosome 10p13A 1998 study done in Britain revealed that children with a mother who had a congenital heart defect (including PAVSD) had a higher risk of being born with a congenital heart defect themselves than those whose father had a congenital heart defect. Syndromes
Some cases of PA(VSD) have been associated with genetic syndromes such as VACTERL association, Alagille syndrome, CHARGE syndrome, trisomy 13, 18, and 21. Environmental
While congenital heart defects cant be acquired, they can also be caused by environmental factors the mother exposed herself to before and/or during pregnancy, these include:
Smoking
Certain medications (e.g.
Exophoria is a form of heterophoria in which there is a tendency of the eyes to deviate outward. During examination, when the eyes are dissociated, the visual axes will appear to diverge away from one another.The axis deviation in exophoria is usually mild compared with that of exotropia. Cause
Exophoria can be caused by several factors, which include:

Refractive errors - distance and near deviation approximately equal. Divergence excess - exodeviation is more than 15 dioptres greater for distance than near deviation. Convergence insufficiency - near exodeviation greater than distance deviation.These can be due to nerve, muscle, or congenital problems, or due to mechanical anomalies. Unlike exotropia, fusion is possible in this condition, causing diplopia to be uncommon. Diagnosis
Prevalence
Exophoria is particularly common in infancy and childhood, and increases with age. References


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