**Core Concept**
The double helix structure of DNA consists of two complementary strands of nucleotides coiled together, with the sugar-phosphate backbone on the outside and nitrogenous bases projecting inward. This structure is crucial for DNA replication and repair, making it susceptible to damage from ionizing radiation.

**Why the Correct Answer is Right**
The double helix is the most susceptible part of DNA to radiation because the nitrogenous bases, which are essential for base pairing and DNA replication, are exposed and vulnerable to ionizing radiation. When radiation interacts with the DNA double helix, it can cause direct damage to the bases, leading to mutations, or indirect damage through the formation of free radicals that can also alter the bases. This susceptibility is due to the high density of nitrogenous bases in the double helix, making it more prone to radiation-induced damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Nucleotides are the building blocks of DNA, but they are not the most susceptible part of DNA to radiation. While nucleotides can be affected by radiation, the double helix structure itself is more vulnerable.
**Option B:** Nucleosides are the components of nucleotides, but they are not the primary target of radiation in DNA.
**Option C:** Histones are proteins that DNA wraps around to form chromatin, but they are not the most susceptible part of DNA to radiation.

**Clinical Pearl / High-Yield Fact**
Ionizing radiation can cause DNA damage by inducing single-strand breaks, double-strand breaks, or base modifications, leading to mutations and potentially cancer. Understanding the susceptibility of the DNA double helix to radiation is crucial in radiology and radiation oncology.

**✓ Correct Answer: D. Double helix**