**Core Concept**
Small round cell tumors (SRCTs) are a group of malignant neoplasms that share similar histological and cytological characteristics, often presenting with rapid growth and aggressive behavior. These tumors typically originate from neuroectodermal cells and can arise in various locations within the body, including the central nervous system, soft tissues, and viscera. The common thread among SRCTs is their primitive, undifferentiated, and often round morphology.

**Why the Correct Answer is Right**
SRCTs are characterized by their high mitotic activity, necrosis, and tendency to invade surrounding tissues. The most common SRCTs include neuroblastoma, Ewing's sarcoma, rhabdomyosarcoma, and medulloblastoma, among others. These tumors often express specific markers such as CD99, CD56, and synaptophysin, which aid in their diagnosis and differentiation from other types of malignancies.

*Option A: Pseudopods*
Pseudopods are characteristic of certain types of neoplastic cells, but they are not a defining feature of small round cell tumors as a whole.

*Option B: Malignant peripheral nerve sheath tumors (MPNSTs)*
MPNSTs are a type of soft tissue sarcoma, but they do not typically exhibit the small round cell morphology characteristic of SRCTs.

*Option C: Melanomas*
Melanomas are a type of skin cancer that can exhibit a variety of histological patterns, but they are not classified as small round cell tumors.

*Option D: Lymphomas*
Lymphomas are a group of cancers that arise from the immune system, but they typically do not exhibit the small round cell morphology of SRCTs.

**Clinical Pearl / High-Yield Fact**
When encountering a small round cell tumor, it is essential to consider the patient's age, location of the tumor, and specific histological features to narrow down the differential diagnosis and guide further testing.

**Correct Answer:** B.