A despesa médica precisa de comprovante de despesa.

Because despesa is a feminine noun in Portuguese. That means it normally takes the article a, not o.

Also, the adjective médica has to agree with despesa, so it is feminine too:

• a despesa médica
• not o despesa médico

This kind of gender agreement is very common in Portuguese.

Adjectives in Portuguese usually agree in gender and number with the noun they describe.

Since despesa is feminine singular, the adjective must also be feminine singular:

• despesa médica
• despesas médicas

If the noun were masculine, then médico would be possible, for example:

• procedimento médico
• atendimento médico

The verb precisar can mean to need, and in this use it normally takes the preposition de:

• precisar de algo = to need something
• precisar de ajuda = to need help
• precisar de um documento = to need a document

So in this sentence:

• A despesa médica precisa de comprovante de despesa.

the structure is:

• subject: A despesa médica
• verb: precisa
• preposition: de
• complement: comprovante de despesa

English says need something with no preposition, but Portuguese often uses precisar de.

In standard Brazilian Portuguese, the safest and most natural form is precisa de um comprovante.

You may sometimes hear people drop the de in casual speech, but for learners and in formal writing, use:

• precisa de comprovante
• precisa de um comprovante

That is the recommended pattern.

Portuguese often leaves out the article when speaking in a general or categorical way.

So:

• precisa de comprovante de despesa

means something like it requires proof of the expense in a general sense.

If you say:

• precisa de um comprovante de despesa

that sounds more like it needs a receipt/proof document, emphasizing one document.

If you say:

• precisa do comprovante de despesa

that usually refers to a specific receipt or proof already known in the context.

So the version without an article is very natural when giving rules, requirements, or general instructions.

Comprovante is a very common Brazilian Portuguese word for a document that proves something.

Depending on context, it can mean:

• receipt
• proof
• voucher
• confirmation slip
• supporting document

Examples:

• comprovante de pagamento = proof of payment / payment receipt
• comprovante de residência = proof of address
• comprovante de renda = proof of income

In your sentence, comprovante de despesa means a document showing that the expense really happened.

They are doing two different jobs.

• despesa médica = the type of expense
• comprovante de despesa = the document proving the expense

So even though despesa appears twice, it is not really redundant in Portuguese. The first tells you what kind of expense it is, and the second is part of a fixed expression meaning proof/receipt of expense.

Portuguese often repeats nouns like this more than English does.

Yes, it is a very natural administrative or financial expression in Brazilian Portuguese.

In practical contexts such as taxes, reimbursements, health plans, or company expenses, Brazilians commonly use combinations like:

• comprovante de despesa
• comprovante de pagamento
• comprovante fiscal
• nota fiscal

Depending on the situation, comprovante de despesa may refer broadly to any document proving the expense, while nota fiscal is specifically an invoice or tax receipt.

Sometimes yes, but despesa is more natural in formal, financial, or administrative contexts.

Compare:

• despesa = expense, often more formal/accounting-like
• gasto = spending/expense, often a bit more general or everyday

For reimbursement, taxes, insurance, or official paperwork, despesa médica sounds very appropriate.

A learner should treat despesa médica as the standard formal expression for medical expense.

Because the subject is singular:

• A despesa médica = singular

So the verb is also singular:

• A despesa médica precisa...

If the subject were plural, the verb would change:

• As despesas médicas precisam de comprovante de despesa.

This is normal subject-verb agreement.

Yes, that is possible, but it changes the meaning slightly.

• As despesas médicas precisam de comprovante de despesa
  = medical expenses require proof of expense, speaking generally

• As despesas médicas precisam de comprovantes de despesa
  = medical expenses require proof documents / receipts, emphasizing multiple documents

Portuguese often allows both singular and plural in these generic requirement-style statements, depending on whether the speaker is thinking of the requirement in general or the actual documents involved.

Yes, especially in formal or bureaucratic contexts, but depending on the situation, a Brazilian might also say something slightly more specific, such as:

• A despesa médica precisa de comprovante.
• É preciso apresentar comprovante da despesa médica.
• Despesas médicas precisam de comprovante.

Your sentence is grammatical and understandable. It sounds like the kind of language used in forms, reimbursement instructions, or tax guidance.

Both can mean needs, and both commonly take de.

• precisa de = more common and everyday
• necessita de = more formal or bureaucratic

So:

• A despesa médica precisa de comprovante de despesa.
• A despesa médica necessita de comprovante de despesa.

Both are correct, but precisa de is usually the more common choice in normal Brazilian Portuguese.