Okay, let's tackle this question about the percentage of HbF in a 6-month-old infant. First, I need to recall what HbF stands for. HbF is fetal hemoglobin. I remember that in fetuses, HbF is the main type of hemoglobin, but after birth, it starts to decrease and gets replaced by adult hemoglobin, which is HbA.

So the question is asking what percentage of HbF is present in a 6-month-old. I think the levels drop over time. At birth, newborns have a high percentage, maybe around 70-90%, and then it decreases. By 6 months, I believe it's significantly lower. Let me think about the timeline.

I recall that by 6 months, HbF drops to about 10-15%, and by 2 years, it's about 2%. So the correct answer here should be around 10-15%. Now, looking at the options (even though they're not provided), the correct answer would be the one that states 10-15%.

The other options might be higher or lower. For example, if an option says 70%, that's more like at birth. If another says 2%, that's more like 2 years old. So the wrong options would be out of the 6-month range.

The core concept here is the physiological switch from fetal to adult hemoglobin. The key is knowing the timeline of HbF decline. The clinical pearl is that HbF levels are high at birth and decrease postnatally, so in a 6-month-old, expecting around 10-15% is crucial for diagnosing conditions like anemia or hemoglobinopathies. Also, in certain diseases like sickle cell, HbF levels might be higher, but that's a different context.

**Core Concept**
Fetal hemoglobin (HbF) predominates in utero but declines postnatally due to increased erythropoietin and decreased placental hormones. By 6 months, **HbF typically constitutes 10–15%** of total hemoglobin, with adult hemoglobin (HbA) becoming dominant.

**Why the Correct Answer is Right**
At 6 months postnatal, the physiological switch from fetal to adult hemoglobin is largely complete. Fetal hemoglobin synthesis declines due to reduced γ-globin gene expression and increased δ- and β-globin synthesis. By this age, HbF levels stabilize at **10–15%**, reflecting the body’s adaptation to oxygen availability outside the placental environment.

**Why Each Wrong Option is Incorrect**
**Option A:** *70–90%* – Represents HbF levels at birth, not 6 months.
**Option B:** *2–5%* – Reflects HbF levels by 2 years of age, not 6 months.
**Option D:** *20–30%* – Overestimates HbF; this range may occur in infants with hemoglobinopathies (e.g., β-thalassemia) but is not normal for 6 months.

**Clinical Pearl**
Remember the **“10–15% rule”** for Hb

✓ Correct Answer: A. 10