**Core Concept**
Seminoma is a type of germ cell tumor that originates from the primordial germ cells in the testes. It is a highly radiosensitive tumor with a good prognosis, especially when diagnosed at an early stage.

**Why the Correct Answer is Right**
Seminomas are characterized by their slow growth rate, and they often present with a painless testicular mass. The cells of seminoma are highly sensitive to radiation therapy, which is a common treatment approach. This sensitivity to radiation is due to the presence of specific genetic mutations that make the cells more vulnerable to radiation-induced DNA damage. The tumor markers alpha-fetoprotein (AFP) and human chorionic gonadotropin (hCG) are typically elevated in non-seminomatous germ cell tumors, but not in pure seminomas.

**Why Each Wrong Option is Incorrect**
**Option A:** Seminomas are indeed highly radiosensitive, which makes this option incorrect. In fact, radiation therapy is a common treatment approach for seminomas.
**Option B:** Pure seminomas are typically associated with elevated levels of lactate dehydrogenase (LDH), not beta-human chorionic gonadotropin (hCG). This makes option B incorrect.
**Option C:** Seminomas are indeed more common in young adults, with a peak incidence in the third and fourth decades of life. This option is correct, but we are looking for an exception.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to confuse seminoma with non-seminomatous germ cell tumors (NSGCTs). Remember that NSGCTs are typically associated with elevated tumor markers such as AFP and hCG, while pure seminomas are not.

**Correct Answer: B.**