**Core Concept**
The underlying condition is a mother with diabetes mellitus (DM), which affects the fetus's metabolic and endocrine systems. The fetus is exposed to high glucose levels in the womb, leading to changes in its physiology.

**Why the Correct Answer is Right**
The correct answer is **A. Hypercalcemia** because babies born to diabetic mothers (BDMs) are more likely to experience hypocalcemia, not hypercalcemia. Hypocalcemia in BDMs is due to the fetus's increased production of calcitonin in response to high glucose levels. Calcitonin helps regulate calcium levels by decreasing bone resorption and increasing calcium excretion in the urine.

**Why Each Wrong Option is Incorrect**
* **Option B:** Hypokalemia is a possible complication in BDMs due to the increased insulin levels in the fetus, which can lead to potassium uptake in cells. However, the most common electrolyte imbalance in BDMs is actually hypokalemia, not hypocalcemia (the correct answer). The question is testing the student's understanding of the most likely electrolyte imbalance in BDMs, which is hypokalemia.
* **Option C:** Hypoglycemia is a common condition in BDMs due to the sudden drop in glucose levels after birth. The fetus's pancreas is adapted to produce insulin in response to high glucose levels, leading to hypoglycemia after birth.
* **Option D:** Obesity is a possible long-term complication of BDMs due to the increased insulin resistance and pancreatic beta-cell adaptation during fetal development.

**Clinical Pearl / High-Yield Fact**
Babies born to diabetic mothers are at increased risk of hypocalcemia, hypoglycemia, and hypokalemia due to the fetal adaptations to the high glucose environment.

**Correct Answer:**
✓ Correct Answer: A. Hypercalcemia