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In the context of congenital cardiac defects in a newborn, particularly when the mother is unimmunized, one must consider the potential impacts of maternal infections such as rubella. Congenital rubella syndrome is known to cause a range of birth defects, including cardiac defects, cataracts, and hearing loss.

Deafness is one of the classic manifestations associated with congenital rubella syndrome. The virus can affect the development of the inner ear structures leading to auditory deficits. This connection between maternal rubella infection and the resulting symptomatology is well-documented and substantiates why deafness would be expected in a newborn with severe cardiac defects from an unimmunized mother.

The other choices reflect conditions that do not necessarily correlate with the information provided in the question scenario. For instance, features such as a web neck (often associated with Turner syndrome) or absence of a thymic shadow (which could indicate DiGeorge syndrome) are not directly linked to the maternal risk factors mentioned. An abnormal karyotype may suggest chromosomal abnormalities but does not specifically relate to the context of maternal immunization status or the potential for congenital rubella. Thus, the association of deafness with congenital cardiac defects in this setting is the most relevant and accurate consideration.