**Core Concept**
Bladder cancer extending muscle deep is considered muscle-invasive bladder cancer (MIBC), a type of non-metastatic bladder cancer. The primary treatment goal for MIBC is to remove the tumor and prevent recurrence. Radical cystectomy, a surgical procedure that involves the removal of the bladder, is a standard treatment for MIBC.

**Why the Correct Answer is Right**
Neoadjuvant chemotherapy followed by radical cystectomy is the best treatment for muscle-invasive bladder cancer. Neoadjuvant chemotherapy is administered before surgery to reduce tumor size and improve surgical outcomes. This approach has been shown to improve survival rates and reduce the risk of recurrence compared to radical cystectomy alone. Radical cystectomy is then performed to remove the bladder and any affected surrounding tissues. This multimodal approach is the standard of care for MIBC.

**Why Each Wrong Option is Incorrect**
**Option A:** Intravesical chemotherapy involves instilling chemotherapy directly into the bladder and is typically used for non-muscle invasive bladder cancer (NMIBC). It is not effective for muscle-invasive bladder cancer and would not be the best treatment for this patient.

**Option C:** Radiotherapy, also known as external beam radiation therapy, can be used as a treatment for bladder cancer, but it is not the best option for muscle-invasive disease. Radiotherapy may be used as an alternative to surgery in certain cases, but it is not as effective as neoadjuvant chemotherapy followed by radical cystectomy.

**Option D:** Transurethral resection (TUR) is a surgical procedure that involves removing bladder tumors through the urethra. While TUR is often used to diagnose and treat non-muscle invasive bladder cancer, it is not sufficient for muscle-invasive bladder cancer and would not be the best treatment for this patient.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the treatment of choice for muscle-invasive bladder cancer has evolved over time, and the use of neoadjuvant chemotherapy followed by radical cystectomy has become the standard of care. This approach has been shown to improve survival rates and reduce the risk of recurrence.

**✓ Correct Answer: B. Neoadjuvant chemotherapy followed by radical cystectomy**