Okay, let's tackle this question about the daily calcium requirement for infants. Hmm, I need to recall the pediatric nutrition guidelines. The core concept here is understanding the calcium needs during different developmental stages, especially in infants because their bones are growing rapidly.

Calcium is crucial for bone development, and infants require it for proper growth. The correct answer is likely related to the recommended dietary allowances (RDAs) set by organizations like the AAP or WHO. Let me think—newborns up to 6 months probably need around 200 mg/day, and then it increases for older infants. Wait, maybe 200 mg for 0-6 months and 260 mg for 7-12 months? I think that's right.

Now, the wrong options might be numbers like 500 mg or 1000 mg, which are higher and more for adolescents or adults. Others might be lower, like 100 mg, which is insufficient. I need to explain why each incorrect option is wrong. For example, 500 mg is excessive for infants and could lead to hypercalcemia. 100 mg is too low and would cause deficiencies.

Clinical pearl: Remember that infant calcium needs are lower than adults because their absorption is more efficient. Also, formula and breast milk provide adequate calcium, so supplementation isn't usually needed unless there's a deficiency. The key takeaway is the RDA numbers for different age groups. Let me structure this into the required sections, making sure each part is concise and covers all points.

**Core Concept**
Calcium requirements in infants are determined by rapid skeletal growth and dietary absorption efficiency. The **Recommended Dietary Allowance (RDA)** for calcium varies by age: **0–6 months (200 mg/day)**, **7–12 months (260 mg/day)**, ensuring adequate bone mineralization without excess.

**Why the Correct Answer is Right**
Infants aged 0–6 months require **200 mg/day** of calcium, primarily obtained from breast milk or formula. Breast milk contains ~30 mg/100 mL, and standard formulas are fortified to meet this need. Excess calcium is avoided to prevent nephrocalcinosis, while deficiency risks rickets. The RDA reflects optimal intake for bone development during this critical period.

**Why Each Wrong Option is Incorrect**
**Option A:** 100 mg/day is insufficient, leading to hypocalcemia and impaired bone growth.
**Option B:** 300 mg/day exceeds requirements, increasing the risk of hypercalciuria and renal damage.
**Option D:** 500 mg/day is excessive for infants; this level is typical for adolescents or adults, not neonates.

**Clinical Pearl / High-Yield Fact**
Remember: **"200 for the first half, 260 for the year."** Infants’ calcium needs are lower than adults due to efficient intestinal absorption (~60% vs. 15–20% in adults). Routine supplementation is unnecessary if fed adequately with breast milk or formula.

**Correct Answer: B. 200 mg/day**

✓ Correct Answer: B. 500 mg