**Core Concept**
A painful breast lump in a lactating mother requires a careful evaluation to distinguish between a benign and malignant lesion. The first investigation is crucial in guiding further management and minimizing unnecessary procedures.

**Why the Correct Answer is Right**
The first step in evaluating a breast lump in a lactating mother is to assess for signs of infection, such as fever, redness, or swelling. However, in the context of lactation, the primary concern is to differentiate between a **galactocele** (a milk-filled cyst) and a **mastitis** (infection of the breast tissue). The clinical evaluation, including a careful history and physical examination, is essential to determine the need for further investigations.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ultrasonography may be useful in further characterizing the breast lump, but it is not the first investigation to be done.
* **Option B:** Mammography is not typically used in lactating women due to the dense breast tissue and the potential for radiation exposure to the fetus.
* **Option D:** Biopsy is an invasive procedure and should not be performed as a first-line investigation.

**Clinical Pearl / High-Yield Fact**
In lactating women, a painful breast lump is often a galactocele, which can be managed conservatively with warm compresses and continued breastfeeding. A thorough clinical evaluation is essential to avoid unnecessary procedures and minimize the risk of complications.

**Correct Answer: C. Clinical evaluation.**