Preciso comparecer por causa do exame toxicológico.

Portuguese often drops subject pronouns when the verb already makes the subject clear.
Preciso already means I need, so Preciso comparecer... is perfectly natural.

You could also say Eu preciso comparecer..., but adding eu usually adds emphasis, contrast, or clarity.

Here, preciso is the 1st person singular form of precisar, meaning to need.

So:

• preciso = I need
• você precisa = you need
• ele/ela precisa = he/she needs

Important: preciso can also be an adjective meaning precise/accurate, but that is not what it means in this sentence.

Because precisar can be used in two main ways:

• precisar + infinitive verb
  
  • Preciso comparecer. = I need to appear/show up.
• precisar de + noun
  
  • Preciso de ajuda. = I need help.

So in your sentence, comparecer is a verb in the infinitive, which is why there is no de.

Comparecer is a somewhat formal verb meaning:

• to appear
• to attend
• to show up
• to be present

In this sentence, it suggests something like I need to show up / I need to attend.

It is more formal than just ir (to go), and it is often used for appointments, official requests, hearings, exams, interviews, and similar situations.

It is common, but it sounds more formal or official than many everyday alternatives.

For example:

• Preciso comparecer amanhã. = more formal
• Preciso ir amanhã. = more casual
• Preciso estar lá amanhã. = very natural in speech

So if this sentence appears in an official or administrative context, comparecer fits very well.

Por causa de is a fixed expression meaning:

• because of
• due to
• on account of

In the sentence, de + o contracts to do:

• por causa de o exame → por causa do exame

So:

• por causa do exame toxicológico = because of the toxicological exam / because of the drug test

Do is a contraction of:

• de + o = do

This kind of contraction is extremely common in Portuguese.

Other examples:

• de + a = da
• de + os = dos
• de + as = das

So do exame literally comes from de o exame, but native speakers use the contracted form do.

Portuguese often uses the definite article where English may or may not use one.

Here, o exame toxicológico suggests a specific, known exam/test:

• the toxicological exam
• the drug test

If you said um exame toxicológico, that would mean a toxicological exam, which sounds less specific.

So the article helps show that this is a particular exam already understood in the context.

Literally, it means toxicological exam/test, but in Brazil it often refers to a drug screening test, especially in administrative, medical, or legal contexts.

So depending on the context, English translations might include:

• toxicology test
• drug test
• drug screening

In real-life Brazilian usage, exame toxicológico often strongly suggests a test for drugs.

Yes, in many situations you can, but the tone changes.

• Preciso comparecer por causa do exame toxicológico.
  More formal; sounds like you must officially show up.
• Preciso ir por causa do exame toxicológico.
  More casual; simply means you need to go.

So both can work, but comparecer sounds more appropriate in formal or official contexts.

Often, yes—it can sound like a reason connected to a problem, obligation, or inconvenience. But it is not always negative.

In this sentence, it sounds natural because the speaker is explaining the reason for needing to attend.

Depending on context, other options could be:

• devido ao exame toxicológico = more formal, due to the toxicological exam
• por conta do exame toxicológico = common in speech, sometimes similar to because of

Still, por causa do is completely normal here.

Yes. Portuguese allows some flexibility.

For example:

• Preciso comparecer por causa do exame toxicológico.
• Por causa do exame toxicológico, preciso comparecer.

Both are correct. The second version emphasizes the reason more strongly.

The original order is very natural and neutral.

In Brazilian Portuguese, it is roughly:

• com-pa-re-CER

The stress is on the last syllable: -cer.

A rough English-friendly approximation is: kohm-pah-reh-SAIR
But the real Portuguese sounds are a bit different, especially the final r, which varies by region.

The stress falls on ló:

• to-xi-co-LÓ-gi-co

That is why it has the written accent: toxicológico.

This accent mark tells you where the stressed syllable is.