**Core Concept**
In a lactating woman presenting with a painful breast lump, the clinical differential diagnosis includes mastitis, abscess, or a benign condition such as a galactocele. The underlying concern is to differentiate between a bacterial infection and a benign process, guiding the choice of investigation.

**Why the Correct Answer is Right**
Ultrasound is the preferred imaging modality for evaluating breast abnormalities in lactating women, as it is non-invasive, does not involve ionizing radiation, and provides accurate information about cystic or solid lesions. In the context of a painful breast lump, ultrasound can help differentiate between a fluid-filled cyst (e.g., galactocele) and a solid mass, which may suggest an abscess or mastitis. Additionally, ultrasound can guide aspiration or drainage of a fluid-filled lesion.

**Why Each Wrong Option is Incorrect**
**Option A:** Mammography is not the preferred initial imaging modality in lactating women due to breast tissue changes and the risk of radiation-induced breast cancer.

**Option B:** MRI is not typically used as an initial imaging modality for breast lumps in lactating women, as it is more expensive and less readily available than ultrasound.

**Option C:** Biopsy may be considered if there is a high suspicion of malignancy or if the clinical presentation is atypical, but it is not the next investigation to be done in a lactating woman with a painful breast lump.

**Clinical Pearl / High-Yield Fact**
In lactating women, a painful breast lump is often due to mastitis or an abscess, which requires prompt antibiotic treatment and may require drainage. Ultrasound is a valuable tool in guiding this management.

**Correct Answer:** C.