## Core Concept
The question pertains to the late side effects associated with the treatment of Hodgkin's disease, a type of lymphoma. Treatment for Hodgkin's disease often involves chemotherapy, radiation therapy, or a combination of both. A significant concern with these treatments is the risk of long-term side effects, which can be severe and life-threatening.

## Why the Correct Answer is Right
The correct answer, **Second Primary Malignancy (SPM)**, is considered a serious late side effect of treating Hodgkin's disease. This is because patients who have undergone chemotherapy and/or radiation therapy for Hodgkin's disease are at an increased risk of developing a second primary malignancy, such as acute myeloid leukemia (AML) or non-Hodgkin lymphoma. This risk arises due to the mutagenic effects of chemotherapy agents and radiation on the patient's DNA, leading to the development of new cancers.

## Why Each Wrong Option is Incorrect
- **Option A:** While **Infertility** is indeed a potential side effect of chemotherapy and radiation, particularly when these treatments are administered to young patients, it is not typically considered as severe or life-threatening as a second primary malignancy. Infertility can significantly impact a patient's quality of life but does not carry the same risk of mortality as SPM.
- **Option B:** **Cardiotoxicity** is another well-recognized side effect of certain chemotherapeutic agents (e.g., anthracyclines) and radiation therapy, especially when directed at the chest. While cardiotoxicity can lead to severe and life-threatening conditions, such as heart failure, advances in treatment protocols and cardioprotective measures have mitigated this risk to some extent.
- **Option C:** **Pulmonary Toxicity** can occur, especially with certain chemotherapeutic agents and radiation to the chest. It can manifest as pneumonitis or fibrosis. Though serious, the risk and severity can often be managed with careful monitoring and adjustment of treatment.

## Clinical Pearl / High-Yield Fact
A critical point to remember is that the risk of **second primary malignancies** increases significantly with time following the initial treatment for Hodgkin's disease. This highlights the importance of long-term follow-up for survivors. A commonly recalled fact is that the risk of SPM, particularly AML, peaks around 5-10 years after treatment, although it can occur even decades later.

## Correct Answer Line
**Correct Answer: D. Second Primary Malignancy.**