Find the definition for Blood Test in Manage | Database | Fields and double click it. Select the validate tab and select the "by calculation" option.
Enter this calculation: IsEmpty ( Pa Inf Maternal )
Then you can also enter a custom validation message such as "Do not enter data in this field if..."
Also, if you have a list of fields for a series of such tests, you may find it better to set up a related table of records for this purpose and place them in a portal.
But what if they have to answer 'Yes, 'No' or 'Don't know' to the first question (Pa Inf Maternal) so it's not that the Pa Inf Maternal field is empty, it will have data in it, but they shouldn't enter data into the 'blood test' field if they've entered 'No' or 'don't know' into the Pa Inf Maternal field.
That's just a slightly different expression: Pa Inf Maternal = "Yes"