Okay, let's tackle this question. The question is about a BCL-2 inhibitor used in CLL. First, I need to recall what BCL-2 does. BCL-2 is an anti-apoptotic protein, so inhibitors would promote cell death in cancer cells. CLL stands for Chronic Lymphocytic Leukemia, which is a type of B-cell malignancy.

Now, the correct answer here should be Venetoclax. Let me confirm. Venetoclax is indeed a BCL-2 inhibitor approved for CLL. It works by binding to BCL-2 and neutralizing its anti-apoptotic effect, leading to programmed cell death.

Looking at the options, even though they aren't listed, I need to address each wrong option. Common BCL-2 inhibitors in other contexts might be Obinutuzumab, but that's a monoclonal antibody against CD20. Ibrutinib is a BTK inhibitor, not BCL-2. Venetoclax is the only BCL-2 inhibitor here.

Clinical pearl: Remember that in CLL, targeting BCL-2 with Venetoclax is especially effective in cases with 17p deletion or TP53 mutation where other therapies may not work. Also, Venetoclax requires a dose-escalation to prevent tumor lysis syndrome.

So the correct answer is Venetoclax. Let me structure the explanation with the required sections, making sure each part is concise and covers the key points without exceeding the character limit.

**Core Concept**
This question tests knowledge of targeted therapies for Chronic Lymphocytic Leukemia (CLL). BCL-2 (B-cell lymphoma-2) is an anti-apoptotic protein overexpressed in CLL, and its inhibition induces cancer cell death via mitochondrial apoptosis pathways.

**Why the Correct Answer is Right**
Venetoclax is a selective BCL-2 inhibitor that binds to the BCL-2 protein, neutralizing its anti-apoptotic effect. This allows pro-apoptotic proteins (e.g., BAX, BAK) to activate, leading to mitochondrial outer membrane permeabilization and caspase-dependent apoptosis. It is FDA-approved for CLL, especially in patients with 17p deletion or TP53 mutations resistant to chemoimmunotherapy.

**Why Each Wrong Option is Incorrect**
**Option A:** Ibrutinib is a Bruton’s tyrosine kinase (BTK) inhibitor used in CLL but acts via a different mechanism (blocking B-cell receptor signaling).
**Option B:** Obinutuzumab is a CD20-targeting monoclonal antibody (anti-CD20) used in CLL; it induces cell death via antibody-dependent cellular cytotoxicity.
**Option D:** Rituximab is another anti-CD20 monoclonal antibody, not a BCL-2 inhibitor.

**Clinical Pearl / High-Yield Fact**
Venetoclax is a cornerstone in high-risk CLL (e.g., 17p deletion) and requires a 2- to 4-week dose-escalation to prevent tumor lysis syndrome. Remember: **"BCL-2 = Venetoclax"** for exam recall.

**Correct Answer: C. Venetoclax**

✓ Correct Answer: D. Venetoclax