**Core Concept:** Pembrolizumab is a monoclonal antibody used as an immune checkpoint inhibitor for the treatment of certain cancers. Immune checkpoint inhibitors work by blocking the interaction between immune checkpoint proteins, such as PD-1 and its ligands PD-L1 and PD-L2, thereby enhancing the anti-tumor immune response.

**Why the Correct Answer is Right:** Pembrolizumab is approved for the treatment of advanced or recurrent non-small cell lung cancer (NSCLC) with high programmed death-ligand 1 (PD-L1) expression. PD-L1 is a ligand that binds to PD-1 and helps tumors evade the immune system. By blocking PD-1/PD-L1 interaction, pembrolizumab enhances the immune system's ability to recognize and eliminate cancerous cells.

**Why Each Wrong Option is Incorrect:**

A. Melanoma (Option A) is another type of cancer that can be treated with pembrolizumab, but the correct answer involves NSCLC (non-small cell lung cancer).
B. Pembrolizumab is not indicated for treating autoimmune diseases or asthma.
C. Pembrolizumab does not target HER2 (human epidermal growth factor receptor 2), which is used to treat certain breast and gastrointestinal cancers.
D. Pembrolizumab is not used for treating solid tumors without high PD-L1 expression, as its primary indication is for advanced NSCLC patients with high PD-L1 expression.

**Clinical Pearl:** Understanding the role of immune checkpoints and PD-L1 expression in cancer progression is crucial for selecting appropriate cancer treatments. Pembrolizumab, as an immune checkpoint inhibitor, is used when cancer cells express high levels of PD-L1 to enhance the immune system's ability to recognize and eliminate cancerous cells.

**Correct Answer:** D. Solid tumors without high PD-L1 expression are not indicated for pembrolizumab treatment, as its primary indication is for advanced NSCLC patients with high PD-L1 expression.