**Core Concept**
The patient's symptoms, including episodic hypertension, headaches, and elevated urinary vanillylmandelic acid (VMA) levels, suggest a diagnosis of pheochromocytoma, a catecholamine-secreting tumor that arises from the adrenal medulla. This condition is characterized by the excessive production of epinephrine and norepinephrine, leading to symptoms such as hypertension, tachycardia, and sweating.

**Why the Correct Answer is Right**
The vast majority of pheochromocytomas are benign, but a significant percentage can be malignant. Malignant pheochromocytomas are often larger than benign tumors and may have a more aggressive clinical course. The elevated VMA levels in this patient's urine are a key diagnostic finding, as VMA is a metabolite of catecholamines that is excreted in the urine. The diagnosis of pheochromocytoma is often confirmed by measuring urinary catecholamine metabolites, such as VMA and metanephrines.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the percentage of malignant pheochromocytomas is not less than 10%. While some sources may suggest a lower percentage, the majority of studies estimate that around 10% of pheochromocytomas are malignant.
* **Option B:** This option is incorrect because the percentage of malignant pheochromocytomas is not greater than 50%. While some malignant pheochromocytomas can be aggressive, the majority of studies estimate that around 10% of pheochromocytomas are malignant.
* **Option D:** This option is incorrect because there is no evidence to suggest that the percentage of malignant pheochromocytomas varies by age.

**Clinical Pearl / High-Yield Fact**
It's worth noting that pheochromocytomas are often diagnosed incidentally during imaging studies for other conditions, and the symptoms can be episodic and unpredictable. The diagnosis of pheochromocytoma requires a high index of suspicion and a thorough evaluation of urinary catecholamine metabolites.

**Correct Answer:** C. 10%