**Core Concept**
Breast malignancy can manifest in various ways on mammography, but certain features are more characteristic of cancer than benign lesions. The mammographic appearance of breast tumors is influenced by their histological type, growth pattern, and vascularity.

**Why the Correct Answer is Right**
Clusters of microcalcification are a hallmark of breast malignancy, particularly in ductal carcinoma in situ (DCIS) or invasive ductal carcinoma. These microcalcifications are usually small, irregular, and clustered, reflecting the abnormal cellular proliferation and necrosis associated with cancer. They are often seen in the subareolar region, which correlates with the high incidence of breast cancer in this area. The presence of clusters of microcalcification on mammography warrants further investigation, including ultrasound and biopsy, to confirm the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Low density lesions are more characteristic of benign breast lesions, such as fibroadenomas or cysts, which often appear as well-defined, rounded masses with smooth borders.

**Option B:** Smooth margins are also more typical of benign lesions, indicating a well-circumscribed border that does not extend into surrounding tissue. In contrast, malignant lesions often have irregular or spiculated margins.

**Option D:** Popcorn calcification is a classic feature of fibroadenoma, a benign breast tumor, and is not associated with malignancy.

**Clinical Pearl / High-Yield Fact**
When interpreting mammograms, remember that clusters of microcalcification are a red flag for breast malignancy, and further investigation is warranted. However, not all clusters of microcalcification are cancerous, and a thorough clinical evaluation, including ultrasound and biopsy, is necessary to confirm the diagnosis.

**✓ Correct Answer: C. Clusters of microcalcification**