**Core Concept**
Tumor markers are substances that can be found in higher-than-normal amounts in the blood, urine, or tissues of some patients with certain types of cancer. They are used to monitor the progression of disease, assess the effectiveness of treatment, and detect recurrence.

**Why the Correct Answer is Right**
The most successful application of tumor markers is in monitoring the effect of treatment and early detection of cancer. This is because tumor markers can be elevated in patients with cancer before symptoms appear, allowing for early detection and treatment. For example, prostate-specific antigen (PSA) is used to monitor the effectiveness of treatment in patients with prostate cancer. Elevated levels of PSA after treatment may indicate recurrence of the disease. Additionally, tumor markers can be used to detect the presence of cancer in patients who are undergoing treatment, allowing for early intervention and improved outcomes.

**Why Each Wrong Option is Incorrect**
**Option A:** Screening in asymptomatic periodicity is not the most successful application of tumor markers because it is not a common practice to screen asymptomatic patients for cancer using tumor markers. While some tumor markers, such as PSA, are used in screening, this is not the primary use of these markers.

**Option C:** Differentiating benign from malignant is not a primary use of tumor markers because most tumor markers are not specific to cancer and can be elevated in patients with benign conditions as well. For example, the tumor marker CA 125 is often elevated in patients with ovarian cancer, but it can also be elevated in patients with other conditions, such as endometriosis.

**Option D:** Staging the extent of disease is not a primary use of tumor markers because staging is typically done using imaging studies, such as CT scans or MRI, rather than tumor markers.

**Clinical Pearl / High-Yield Fact**
Tumor markers should not be used as a sole diagnostic tool, but rather as a complementary tool to aid in diagnosis and monitoring of cancer.

**✓ Correct Answer: B. Effect of treatment and early detection**