**Core Concept**
Radiation tolerance is a critical concept in radiation oncology, referring to the maximum dose of ionizing radiation that a particular tissue can withstand without suffering significant damage. This dose varies depending on the type of tissue, with some tissues being more resistant to radiation than others.

**Why the Correct Answer is Right**
The maximum radiation dose tolerable tissue is typically considered to be the spinal cord, with a dose of around 50-60 Gy (5000-6000 rads). This is because the spinal cord is a highly sensitive structure, and doses above this level can cause radiation myelitis, a condition characterized by inflammation and damage to the spinal cord. The spinal cord's sensitivity to radiation is due to the presence of rapidly dividing cells, such as oligodendrocytes, which are responsible for myelinating nerve fibers.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the skin is not the maximum radiation dose tolerable tissue. While the skin can tolerate relatively high doses of radiation, it can still suffer from radiation dermatitis, a condition characterized by inflammation and damage to the skin.
* **Option B:** This option is incorrect because the brain is not the maximum radiation dose tolerable tissue. While the brain is sensitive to radiation, it is not as sensitive as the spinal cord, and doses above 60 Gy (6000 rads) can cause radiation necrosis, a condition characterized by cell death and tissue damage.
* **Option C:** This option is incorrect because the lung is not the maximum radiation dose tolerable tissue. While the lung can tolerate relatively high doses of radiation, it can still suffer from radiation pneumonitis, a condition characterized by inflammation and damage to the lung tissue.

**Clinical Pearl / High-Yield Fact**
When treating tumors with radiation, it's essential to carefully balance the dose of radiation to the tumor with the dose to surrounding normal tissues. The spinal cord is a critical structure to protect, as damage can lead to long-term neurological deficits.

**Correct Answer: D. Spinal cord.**