All of the following are increased in pheochromocytomas. except
**Core Concept**
Pheochromocytomas are rare, catecholamine-secreting tumors that arise from the adrenal medulla. The excess production of catecholamines, including epinephrine, norepinephrine, and dopamine, leads to various systemic effects.
**Why the Correct Answer is Right**
The correct answer is the option that is not typically increased in pheochromocytomas. Catecholamines are produced through a series of enzyme-catalyzed reactions, starting from the amino acid tyrosine. The key enzymes involved are tyrosine hydroxylase, dopa decarboxylase, aromatic L-amino acid decarboxylase, and dopamine beta-hydroxylase. In pheochromocytomas, the excess production of catecholamines leads to increased levels of metanephrines, vanillylmandelic acid (VMA), and homovanillic acid (HVA). The correct answer is the option that is not a metabolite of catecholamines.
**Why Each Wrong Option is Incorrect**
* **Option A:** Metanephrines are increased in pheochromocytomas due to the excess production of epinephrine and norepinephrine. Metanephrines are the O-methylated metabolites of epinephrine and norepinephrine.
* **Option B:** Vanillylmandelic acid (VMA) is a metabolite of catecholamines and is typically increased in pheochromocytomas. VMA is produced through the hydroxylation of epinephrine and norepinephrine.
* **Option C:** Homovanillic acid (HVA) is a metabolite of dopamine and is typically increased in pheochromocytomas. HVA is produced through the hydroxylation of dopamine.
**Clinical Pearl / High-Yield Fact**
The diagnosis of pheochromocytoma is often challenging due to the non-specific symptoms and signs of catecholamine excess. However, a 24-hour urine collection for metanephrines and VMA can be a useful screening test for pheochromocytoma.
**Correct Answer:** D.