Question: Which  international border is San Ysidro Port of Entry at  

Context paragraphs:
Immigration sign: The immigration sign is an American highway safety sign warning motorists to avoid immigrants darting across the road.  It depicts a man, woman, and child with pigtails running.  The signs were erected in response to over one hundred immigrant deaths due to traffic strikes from 1987 to 1990 in two corridors along Interstate 5 along the San Ysidro Port of Entry at the Mexico-United States border and approximately 50 mi north at the San Clemente United States Border Patrol checkpoint in Camp Pendleton.

Mexico–United States border: The Mexico–United States border is an international border separating Mexico and the United States, extending from the Pacific Ocean to the west and Gulf of Mexico to the east.  The border traverses a variety of terrains, ranging from major urban areas to uninhabitable deserts.  s of 2010 , approximately 350 million legal crossings occur annually, and is the most frequently crossed border in the world.

San Ysidro Port of Entry: The San Ysidro Port of Entry is the largest land border crossing between San Diego and Tijuana, and the busiest land border crossing in the world.  It connects Mexican Federal Highway 1 with Interstate 5 on the U.S. side.  The San Ysidro Port of Entry is one of three ports of entry in the San Diego–Tijuana metropolitan region.

Interstate 805: Interstate 805 (I-805) is a major north–south Interstate Highway in Southern California.  It is a bypass of I-5, running roughly through the center of the Greater San Diego region from San Ysidro (part of the city of San Diego) near the Mexico–U.S. border to near Del Mar.  The southern terminus of I-805 at I-5 in San Ysidro is less than a mile north of the Mexican border.  I-805 then traverses the cities of Chula Vista and National City before reentering San Diego.  The freeway passes though the San Diego neighborhoods of North Park, Mission Valley, Clairemont, and University City before terminating at I-5 in the Sorrento Valley neighborhood near the Del Mar city limit.

Otay Mesa Port of Entry: The Otay Mesa Port of Entry is one of three ports of entry (POE) in the San Diego–Tijuana metropolitan region, connecting Otay Mesa in the City of San Diego with the Otay Centenario borough of Tijuana.  The facility was opened in 1983, and was constructed primarily to divert growing commercial truck traffic from the busy San Ysidro Port of Entry.  Since then, significant passenger vehicle and pedestrian traffic has grown as development in the area around the crossing has grown.  Commercial importations through Otay Mesa accounts for billions of dollars worth of freight.

San Ysidro, San Diego: San Ysidro (] ) is a district of the City of San Diego, immediately north of the U.S.-Mexico border.  It neighbors Otay Mesa West to the north, Otay Mesa to the east, and Nestor and the Tijuana River Valley to the west; together these communities form South San Diego, a pene-exclave of the City of San Diego, thus making it possible to travel (by water) between central San Diego and South San Diego without ever leaving the city limits.  Major thoroughfares include Beyer Boulevard and San Ysidro Boulevard.

Tecate Port of Entry: The Tecate Port of Entry is one three ports of entry in the San Diego–Tijuana metropolitan region.  The land port is located between Tecate, California in San Diego County's Mountain Empire and Tecate Municipality in Baja California.  It connects California State Route 188 with Paseo Lazero Cardenas, a spur of Mexico Federal Highway 2, as well as Federal Highway 3 to the south.  It is a minor port in comparison to the larger San Ysidro Port of Entry and Otay Mesa Port of Entry.  This is attributed in part to the fact that reaching the crossing on the US side requires driving on narrow, winding mountain roads.

Tecate: Tecate (    ) is a small city in Baja California, Mexico, and the municipal seat of Tecate Municipality.  It is on the border with Tecate, California, United States in the San Diego–Tijuana metropolitan area.  There is a small port of entry between the sister cities that serves as a calmer alternative to the bustling port of Tijuana.  Although the city is only 64 km from San Diego, the road to the border crossing traverses mountainous terrain, reducing the viability of Tecate as an alternative to the San Ysidro Port of Entry.  As of 2010, the city of Tecate had a population of 64,764.

San Ysidro Transit Center: San Ysidro Transit Center is a San Diego Trolley station on the Blue Line and the southern terminus for this line.  The previous station northwest is Beyer Boulevard.  South of this station is the San Ysidro Port of Entry at the Mexico–United States border.  The station serves primarily as a way to provide access to downtown for the thousands of international commuters and tourists who travel between San Diego County and Tijuana, Mexico.  Its secondary purpose is to provide access to the large shopping areas, including the Las Americas Premium Outlets which are connected to the stop via a pedestrian walkway.  An intercity bus station is located adjacent to the station.

Las Americas Premium Outlets: Las Americas Premium Outlets is a 560000 sqft outlet mall in San Ysidro, San Diego, California located directly on the Mexico-United States border just west of the San Ysidro Port of Entry at the new PedWest crossing from Tijuana to Virginia Avenue on the U.S. side.

Supporting paragraphs: San Ysidro Transit Center Mexico–United States border
Answer: The Mexico–United States border