Hi Amber,
"Extremely ill 7-month-old male child with respiratory failure, respiratory synctyial virus (RSV), pneumonia and Down’s syndrome who requires central venous line placement because of
a? premorbid condition."
The 'a' if you're saying that he requires the line placement because of a condition he suffered from before this one. Also, depending on what this is written for:
"Extremely ill 7-month-old male child suffering from respiratory failure, respiratory synctyial virus (RSV), pneumonia and Down’s syndrome. Due to a premorbid condition the child will require a central venous line placement."
Also:
Spaces, commas and punctuation Some rules, including commas Using commas before 'and'