**Core Concept:** The parameters mentioned in the question are related to maternal and fetal physiological changes during full-term pregnancy. These parameters help in assessing the overall health and wellbeing of the mother and the fetus.

**Why the Correct Answer is Right:** At full-term pregnancy, the key parameters that are typically elevated are:

1. **Option D:** Hematocrit (Hct) - This is the percentage of red blood cells in the maternal blood, reflecting the increased red blood cell mass to meet the increased oxygen demand during pregnancy.
2. **Option C:** Platelet count - Platelet count is usually elevated due to increased production in the bone marrow to support hemostasis and tissue repair during pregnancy.
3. **Option B:** Serum osmolality - Osmolality is the measure of the solute concentration in a solution, here serum (blood) osmolality is increased due to the increased osmoreceptors in the hypothalamus responding to the elevated osmolality in the maternal blood.

But, the correct answer, **Option A:** Blood urea nitrogen (BUN) - Blood urea nitrogen is a marker of renal function. Although pregnancy can affect renal function, the question assumes a healthy mother and fetus. In a healthy individual, BUN levels tend to be slightly elevated during pregnancy due to increased protein intake and reduced renal clearance. However, in this scenario, BUN levels are not significantly elevated.

**Why Each Wrong Option is Incorrect:**

1. **Option D (Hematocrit)**: Although hematocrit is elevated due to increased red blood cell mass, it does not reach such extreme levels as Option D suggests. A normal hematocrit range during pregnancy is 35-45%.
2. **Option C (Platelet Count)**: While platelet count can be mildly elevated in pregnancy, it is unlikely to be as high as Option C suggests (>500,000/mm³).
3. **Option A (Blood Urea Nitrogen)**: Option A suggests that BUN levels are significantly elevated, which may indicate renal dysfunction or proteinuria, neither of which is the case in a healthy pregnancy.

**Clinical Pearl:** BUN levels are more likely to be influenced by protein intake and renal function rather than pregnancy itself. This highlights the importance of interpreting clinical findings in the context of the patient's overall health status and not solely relying on the diagnosis of pregnancy.

In a healthy pregnancy, the elevated levels of hematocrit, platelet count, and BUN are expected due to physiological changes in the maternal body. However, the levels suggested in the wrong options are unrealistically high, which may indicate renal dysfunction or proteinuria, which is not typical of a healthy pregnancy.