**Core Concept**
Mammography is a crucial imaging modality in the evaluation of breast lumps, allowing for the visualization of breast tissue and detection of abnormalities such as masses, calcifications, and architectural distortions.

**Why the Correct Answer is Right**
Mammography is particularly useful in women over 40 years old due to its ability to detect calcifications and subtle changes in breast tissue that may be associated with malignancy. It is also useful in women with dense breast tissue, as it can help differentiate between glandular and fatty tissue. The mammography machine compresses the breast, providing a clear image of the underlying tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is useful for differentiating between solid and cystic lesions, but it may not provide sufficient detail for the evaluation of architectural distortions or calcifications.
**Option B:** MRI is more sensitive than mammography for the detection of breast cancer, particularly in women with dense breast tissue or those with a high-risk genetic profile. However, it is more expensive and not as widely available as mammography.
**Option C:** CT scan is not typically used for the evaluation of breast lumps, as it does not provide sufficient resolution for the visualization of breast tissue.
**Option D:** PET scan is useful in the staging of breast cancer, but it is not the primary imaging modality for the evaluation of breast lumps.

**Clinical Pearl / High-Yield Fact**
A key factor in the evaluation of breast lumps is the age of the patient, with mammography being the primary imaging modality for women over 40 years old and ultrasound being more useful for women under 40 years old.

**Correct Answer: B. MRI.**