Intrauterine Pregnancy (IUP): Given the presence of a hypoechoic structure in the endometrial canal measuring approximately 1.8 cm, likely representing a gestational sac, and a small cystic structure within it, possibly a yolk sac, along with a positive beta-hCG level of 21,251 at 5.5 weeks, an intrauterine pregnancy is the most likely diagnosis. The size of the gestational sac and the presence of a possible yolk sac are consistent with early pregnancy development.

Ectopic Pregnancy: Although less likely than an intrauterine pregnancy given the ultrasound findings, ectopic pregnancy remains a consideration, especially if the patient presents with symptoms such as abdominal pain or if the beta-hCG levels do not rise appropriately. The absence of a definitive fetal pole and the presence of a possible gestational sac in the uterus do not entirely rule out an ectopic pregnancy.
Pregnancy Failure (Miscarriage): The inability to identify a fetal pole at 5.5 weeks, despite a gestational sac being present, raises concerns about the viability of the pregnancy. Pregnancy failure could be a consideration if follow-up ultrasounds do not show appropriate development.

Ectopic Pregnancy with Rupture: This is a life-threatening condition that requires immediate medical attention. Even though the ultrasound does not currently show evidence of an ectopic pregnancy, the possibility cannot be entirely excluded without further follow-up. Symptoms such as severe abdominal pain, vaginal bleeding, or signs of hemodynamic instability would necessitate urgent evaluation.
Molar Pregnancy: Although less common, a molar pregnancy could present with a positive beta-hCG and abnormal ultrasound findings. The absence of a fetal pole and the presence of a cystic structure within the gestational sac could potentially be seen in a molar pregnancy, though the current description is more suggestive of a normal early pregnancy.

Heterotopic Pregnancy: This is a rare condition where there is both an intrauterine and an ectopic pregnancy. Given the presence of a possible gestational sac in the uterus, this diagnosis is less likely but could be considered if the patient is undergoing fertility treatments, which increase the risk of heterotopic pregnancies.
Pseudogestational Sac (Decidua Reaction): In the setting of an ectopic pregnancy, a pseudogestational sac can form within the uterus due to decidual reaction. This would be a rare finding and typically associated with an ectopic pregnancy, which currently seems less likely given the ultrasound description.