**Core Concept**
The distinction between carcinoma in situ (CIS) and invasive carcinoma is crucial in cancer diagnosis, as it determines the stage and prognosis of the disease. Carcinoma in situ refers to a non-invasive form of cancer where the tumor cells are confined within the epithelial layer, whereas invasive carcinoma involves the breach of the basement membrane, allowing cancer cells to invade adjacent tissues.

**Why the Correct Answer is Right**
The presence of a breach in the basement membrane is the key feature that differentiates invasive carcinoma from carcinoma in situ. The basement membrane is a thin, fibrous layer that separates the epithelial layer from the underlying connective tissue. Invasive carcinoma cells have the ability to break through this membrane, allowing them to invade and metastasize to distant sites. This process is facilitated by the loss of cell-cell adhesion molecules and the acquisition of invasive properties by cancer cells. The breach in the basement membrane is a critical step in the progression of cancer from a localized, non-invasive disease to a more aggressive, invasive form.

**Why Each Wrong Option is Incorrect**
**Option A:** Metastasis is a consequence of invasive carcinoma, but it is not the primary feature that differentiates CIS from invasive carcinoma. Metastasis occurs after the cancer cells have breached the basement membrane and invaded adjacent tissues.

**Option B:** Anaplasia refers to the loss of cellular differentiation and organization, which is a feature of many cancers, including CIS and invasive carcinoma. However, anaplasia is not the key feature that distinguishes CIS from invasive carcinoma.

**Option C:** The number of mitotic spindles is a measure of cellular proliferation, which can be increased in both CIS and invasive carcinoma. However, it is not a reliable feature to distinguish between these two conditions.

**Clinical Pearl / High-Yield Fact**
When differentiating between carcinoma in situ and invasive carcinoma, it is essential to examine the histological features of the tumor under a microscope, including the presence or absence of a breach in the basement membrane. This distinction is critical for determining the stage and prognosis of the disease, as well as guiding treatment decisions.

**✓ Correct Answer: D. Breach in basement membrane**