Перед медосмотром медсестра сказала прийти натощак и не пить сладкий чай.

Because перед normally requires the instrumental case when it means before or in front of.

So:

• медосмотр = medical examination
• перед медосмотром = before the medical examination

This works both in time and space:

• перед домом = in front of the house
• перед уроком = before the lesson
• перед медосмотром = before the medical exam

The ending -ом shows the instrumental singular of a masculine noun.

Медосмотр is short for медицинский осмотр, meaning a medical examination, checkup, or medical screening.

It is a common everyday word, especially in clinics, schools, workplaces, and official settings.

Breakdown:

• мед- comes from медицинский = medical
• осмотр = examination, inspection

So медосмотр is a compact, very natural Russian word for a medical check.

In Russian, verbs like сказать, просить, велеть, посоветовать are often followed directly by an infinitive to express telling or advising someone to do something.

So:

• медсестра сказала прийти = the nurse said to come

Literally, it is more like:

• the nurse said to come

Russian does not need a separate word equivalent to English to in this structure beyond the infinitive ending itself.

Very similar examples:

• Он сказал подождать. = He said to wait.
• Врач посоветовал больше спать. = The doctor advised sleeping more.
• Мама велела убрать комнату. = Mom told me to clean the room.

Russian often leaves out an object if it is clear from context.

So медсестра сказала прийти натощак can mean:

• the nurse told me
• the nurse told us
• the nurse told the patient

The sentence does not specify, because Russian frequently omits things that are already understood.

If you wanted to make it explicit, you could say:

• Медсестра сказала мне прийти натощак. = The nurse told me to come on an empty stomach.
• Медсестра сказала пациенту прийти натощак. = The nurse told the patient to come on an empty stomach.

Прийти is the perfective form, and приходить is the imperfective form.

Here, прийти is used because the nurse is talking about one specific occasion: arriving for the exam. The focus is on the completed result of coming.

• прийти = to come, to arrive once, successfully
• приходить = to come regularly, habitually, or as a process

So:

• сказала прийти натощак = said to come/arrive on an empty stomach

If you used приходить, it would sound more like repeated or habitual coming, which does not fit this situation as well.

Натощак means on an empty stomach.

It is an adverb, so it does not change form.

Common medical uses:

• сдавать кровь натощак = to give a blood sample on an empty stomach
• прийти натощак = to come on an empty stomach
• принимать лекарство натощак = to take medicine on an empty stomach

It is a very common word in medical instructions.

This is about aspect again.

• пить = imperfective, to drink
• выпить = perfective, to drink up / drink once

In instructions and prohibitions, Russian often prefers the imperfective when talking about what should not be done in general during a period of time.

So:

• не пить сладкий чай = not to drink sweet tea

This sounds like a general instruction before the exam.

If you said не выпить сладкий чай, it would sound more like avoiding one single completed act of drinking a sweet tea, which is less natural here.

Because чай is the direct object of пить, so it goes in the accusative case.

For masculine inanimate nouns, the accusative is usually the same as the nominative.

So:

• nominative: сладкий чай
• accusative: сладкий чай

That is why the form does not visibly change.

Compare with a feminine noun, where you would see a clearer accusative ending:

• пить сладкую воду = to drink sweet water

И simply joins two infinitive instructions:

• прийти натощак
• не пить сладкий чай

So the nurse said two things:

1. come on an empty stomach
2. do not drink sweet tea

Both infinitives depend on сказала.

You can think of the structure as:

• медсестра сказала
  
  • прийти натощак
  • и не пить сладкий чай

Yes. Russian word order is flexible, though not random.

The original sentence is neutral and natural:

• Перед медосмотром медсестра сказала прийти натощак и не пить сладкий чай.

But other orders are possible, depending on emphasis:

• Медсестра перед медосмотром сказала прийти натощак и не пить сладкий чай.
• Медсестра сказала перед медосмотром прийти натощак и не пить сладкий чай.

The original version starts with перед медосмотром, which sets the time context first: before the medical exam.

That is a very common Russian pattern.

Натощак mainly means without eating, though in medical contexts it often also suggests avoiding certain drinks.

However, doctors and nurses often give extra explicit instructions to avoid misunderstanding. So adding не пить сладкий чай makes the rule clearer.

In practice:

• натощак = on an empty stomach
• не пить сладкий чай = specifically do not drink sweet tea

This extra detail is especially natural in a medical context, because sweet drinks can affect test results.