Looking on the internet deeply has found these results:

a US citizen is the most popular phrase on the web.

an US citizen 150,300 results on the web	a US citizen 14,280,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	Erm, the constitution of the United States. I swear to be a US citizen. The United States has asked for a reextradition clause to be included in the agreement that would prevent an EU Member State sending a US citizen to the International Criminal Court or another international tribunal without its express agreement. I am a US citizen, and I will not be silenced! I'm submitting to the courts the findings of the immigration judge which states that Officer Delko was and always has been a US citizen. I'm a US citizen, I have rights. At that point I didn't see how I could not document him as a US citizen. For starters, a centerpiece of Mike Lee's stump speech is effectively eliminating the part of the 14th Amendment that says a citizen born in the US is a US citizen. So, I've recently become a US citizen.

an US citizen

150,300 results on the web

a US citizen

14,280,000 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

Erm, the constitution of the United States. I swear to be a US citizen.
The United States has asked for a reextradition clause to be included in the agreement that would prevent an EU Member State sending a US citizen to the International Criminal Court or another international tribunal without its express agreement.
I am a US citizen, and I will not be silenced!
I'm submitting to the courts the findings of the immigration judge which states that Officer Delko was and always has been a US citizen.
I'm a US citizen, I have rights.
At that point I didn't see how I could not document him as a US citizen.
For starters, a centerpiece of Mike Lee's stump speech is effectively eliminating the part of the 14th Amendment that says a citizen born in the US is a US citizen.
So, I've recently become a US citizen.