**Core Concept**
The patient's extremely high LDL cholesterol level (>300 mg/dl) is indicative of severe hypercholesterolemia, which can lead to premature atherosclerotic cardiovascular disease (ASCVD). This clinical scenario is often associated with the presence of xanthomas, which are deposits of cholesterol-rich lipids in the skin and tendons.

**Why the Correct Answer is Right**
The presence of xanthomas is a hallmark of untreated or undertreated hypercholesterolemia. These lesions are composed of foam cells, which are macrophages laden with lipids. The accumulation of lipids in the skin and tendons is due to the deposition of cholesterol esters and triglycerides. In this patient, the high LDL level is likely to be the primary driver of xanthoma formation.

**Why Each Wrong Option is Incorrect**
**Option A:** Corneal arcus is a sign of hyperlipidemia, but it is more commonly associated with high levels of HDL cholesterol. While corneal arcus can be seen in patients with hypercholesterolemia, it is not the most likely finding in this patient with extremely high LDL levels.
**Option B:** Palpable pulses are not directly related to hypercholesterolemia or xanthoma formation. This finding is more commonly associated with conditions such as aortic stenosis or peripheral artery disease.
**Option C:** X-ray findings of bone density changes are not typically associated with hypercholesterolemia. While certain lipid disorders can increase the risk of osteoporosis, this is not the most likely finding in this patient.

**Clinical Pearl / High-Yield Fact**
In patients with untreated or undertreated hypercholesterolemia, xanthomas can be a valuable clinical clue to the presence of lipid disorders. These lesions can be found on the elbows, knees, and buttocks, and are often associated with a history of premature cardiovascular disease.

**Correct Answer:** C.