Final answer:
The hormone screened in a urine pregnancy test is human chorionic gonadotropin (hCG), essential for maintaining the early stages of pregnancy by preserving the corpus luteum which produces progesterone.
Step-by-step explanation:
When a medical assistant is collecting a urine pregnancy test from a patient, the hormone that is being screened for is human chorionic gonadotropin (hCG). Human chorionic gonadotropin is a hormone produced by the chorion, which is part of the developing embryo. The presence of hCG in urine or blood is an indicator of pregnancy because it is released by the trophoblastic cells of the fertilized egg (or zygote) shortly after it has implanted in the uterine wall. Specifically, the outer layer of the blastocyst, once a fertilized egg becomes a blastocyst, starts to secrete hCG, which is very similar to LH and helps to preserve the corpus luteum. The continued production of progesterone by the corpus luteum is essential in maintaining the pregnancy in its early stages until the placenta takes over this function.