**Core Concept**
High-risk breast cancer refers to breast cancer that occurs in individuals with a higher likelihood of developing the disease, often due to genetic mutations, family history, or other risk factors. In these cases, the choice of imaging investigation is critical to detect cancer at an early stage.

**Why the Correct Answer is Right**
The investigation of choice for high-risk breast cancer females is MRI (Magnetic Resonance Imaging). This is because MRI provides higher sensitivity and specificity for detecting breast cancer, particularly in dense breast tissue, which is common in high-risk individuals. MRI can detect cancers as small as 2-3 mm and is also effective in evaluating the extent of disease in the breast and assessing the risk of developing cancer in the other breast. The American Cancer Society recommends annual MRI screening for women with a 20-25% or greater lifetime risk of developing breast cancer.

**Why Each Wrong Option is Incorrect**
**Option B:** USG (Ultrasonography) is not the investigation of choice for high-risk breast cancer females. While USG is useful in evaluating palpable breast masses, it is not as sensitive as MRI for detecting cancers in high-risk individuals.

**Option C:** Mammography is the primary screening tool for breast cancer in average-risk women. However, it is not the investigation of choice for high-risk breast cancer females due to its lower sensitivity in dense breast tissue.

**Option D:** Clinical examination is an essential part of breast cancer evaluation but is not sufficient for high-risk individuals. A clinical examination can only detect palpable masses and may not detect cancers that are not palpable.

**Clinical Pearl / High-Yield Fact**
High-risk breast cancer females should undergo annual MRI screening, in addition to clinical examination and mammography, to detect cancer at an early stage. A high-risk individual is defined as someone with a 20-25% or greater lifetime risk of developing breast cancer.

**✓ Correct Answer: A. MRI**