**Core Concept**
Tendon xanthoma is a type of cutaneous manifestation associated with elevated levels of low-density lipoprotein (LDL) cholesterol in the blood. It is characterized by the deposition of cholesterol-rich foam cells in the tendons, leading to their thickening and hardening.

**Why the Correct Answer is Right**
Tendon xanthoma is a specific finding associated with familial hypercholesterolemia (FH), a genetic disorder caused by mutations in the LDL receptor gene. In patients with FH, the LDL receptor is either absent or defective, leading to severely elevated levels of LDL cholesterol in the blood. The high levels of LDL cholesterol in the blood lead to the accumulation of cholesterol in the tendons, resulting in the formation of tendon xanthoma. This condition is often seen in individuals with very high levels of LDL cholesterol, such as the patient in the question with LDL 600mg/dl.

**Why Each Wrong Option is Incorrect**

* **Option B:** Planar xanthoma is a type of xanthoma associated with beta-lipoproteinemia, which is characterized by elevated levels of triglycerides in the blood. This condition is not typically associated with elevated LDL cholesterol levels.
* **Option C:** Lipemia retinalis is a condition characterized by a yellowish discoloration of the retinal vessels due to the presence of high levels of triglycerides in the blood. While it can be associated with elevated triglyceride levels, it is not typically associated with elevated LDL cholesterol levels.
* **Option D:** Tuberoeruptive xanthoma is a type of xanthoma associated with eruptive xanthoma, which is often seen in patients with severe hypertriglyceridemia. This condition is not typically associated with elevated LDL cholesterol levels.

**Clinical Pearl / High-Yield Fact**
Tendon xanthoma is a specific clinical finding that can help diagnose familial hypercholesterolemia, a genetic disorder that can lead to premature cardiovascular disease if left untreated. Early recognition and treatment of this condition can help prevent long-term cardiovascular complications.

**✓ Correct Answer: A. Tendon xanthoma**