**Core Concept**
Cleft lip repair is a surgical procedure aimed at correcting congenital anomalies of the upper lip and primary palate. The timing and approach of cleft lip repair are critical in achieving optimal aesthetic and functional outcomes.

**Why the Correct Answer is Right**
The optimal age for cleft lip repair is between 3 to 6 months of age. This timing is based on the concept of the "golden period" of facial growth, where the lip and palate are still in a relatively pliable state, making it easier to achieve a good repair. Additionally, early repair is associated with improved speech outcomes, reduced risk of secondary deformities, and enhanced social and emotional development.

**Why Each Wrong Option is Incorrect**
**Option A:** Repairing a cleft lip at birth is generally not recommended, as the tissues are still too fragile and may lead to scarring and poor aesthetic outcomes.
**Option B:** Delaying cleft lip repair beyond 6 months of age may result in increased risk of speech difficulties, nasal deformities, and other secondary complications.
**Option C:** Repairing a cleft lip in a single stage is not always possible, especially in cases with significant tissue deficiency or other associated anomalies.

**Clinical Pearl / High-Yield Fact**
The Millard rotation-advancement repair is a widely used technique for cleft lip repair, which involves rotating the lateral lip flap to match the medial lip, thereby achieving a more natural aesthetic outcome.

**Correct Answer:** C. 3 to 6 months of age.