Parallelism Answer 6

6. Fainting can result from a lack of oxygen and a loss of blood.

Here the second noun must be changed to reflect the generic noun form of "lack" in the first half. Previously, the second item had been "losing blood", which is a gerund phrase. One generally does not make a parallel structure with these two types of nouns.

Here is a variant form:

6. Fainting can result from lacking oxygen and losing blood.

Here the parallel structure is maintained by changing the generic noun from "lack" to the gerund phrase form "lacking".