Bij aanhoudende koorts controleert de arts het bloed.

Here bij does not mean a physical location such as next to.

In this sentence, bij means something like:

• in cases of
• when there is
• with

So bij aanhoudende koorts introduces the medical situation or condition.

Because this is a general medical statement.

Dutch often leaves out the article with conditions or substances when speaking generally. So:

• bij aanhoudende koorts = in cases of persistent fever

If you were talking about one specific fever already mentioned, an article could appear in a different sentence, but here the article-less form is the natural one.

Aanhoudende means persistent, continuing, or ongoing.

It comes from the verb aanhouden, which can mean to continue or to persist, depending on context.

So aanhoudende koorts is fever that does not go away quickly.

Because it is an adjective placed before a noun.

The basic adjective form is:

• aanhoudend

But before a noun, Dutch adjectives usually take -e:

• aanhoudende koorts

Compare:

• De koorts is aanhoudend. → adjective after the verb
• de aanhoudende koorts → adjective before the noun

This is because of the Dutch V2 rule: in a main clause, the finite verb must be in the second position.

The first position here is taken by:

• Bij aanhoudende koorts

So the verb has to come next:

• Bij aanhoudende koorts controleert de arts het bloed.

That pushes the subject de arts to a position after the verb.

A more neutral order would be:

• De arts controleert het bloed bij aanhoudende koorts.

Both are grammatical, but the given sentence emphasizes the condition first.

Controleert is the present tense, third-person singular form of controleren.

Examples:

• ik controleer
• jij controleert
• hij controleert
• de arts controleert

Since de arts is singular, controleert is the correct form.

Not usually. This is a very important false-friend point.

In Dutch, controleren often means:

• to check
• to inspect
• to examine

So here controleert het bloed means checks/examines the blood, not controls the blood.

Because bloed is a het-word in Dutch.

So the correct definite article is:

• het bloed

This is just something you have to learn with the noun:

• het bloed
• het hart
• de arts
• de koorts

In context, it usually means the patient’s blood or the blood being examined.

Dutch often uses the definite article in places where English may sound more natural with something like:

• the blood
• the patient’s blood
• a blood sample

So the Dutch wording is normal, even if English might phrase it a little differently.

It is very close in meaning, yes.

In practical English, this phrase often works like:

• if there is persistent fever
• when someone has persistent fever
• in cases of persistent fever

But grammatically, bij is still a preposition, not a conjunction like als.

Yes, often you could.

• arts is a bit more formal or professional
• dokter is more everyday language

So:

• de arts controleert het bloed
• de dokter controleert het bloed

Both are possible, but arts sounds slightly more formal and medical.

Normally, no.

Dutch does not usually put a comma after a short introductory phrase like this. So the standard form is:

• Bij aanhoudende koorts controleert de arts het bloed.

A comma would only appear in unusual stylistic cases, not in normal writing.