Farewell, UCD label

Question: Do all urea cycle disorders result in excess plasma ammonia?

Answer: yes.  Which kind of makes sense, but I had to confirm it.

So, where to from here?  We discard the "UCD" label, and start looking at the broader category of protein metabolism disorders.  That can probably be referred to just as "amino acid disorders", since they describe problems that happen after proteins have been broken down.  Best known is PKU, but I was screened for that at birth.  There is also Maple Syrup Urine Disease (BCKD deficiency), which is an inability to process the valine family (valine, leucine, isoleucine) which is common in animal products.

Those are two of them.  There's a whole host of others; understandable, given how many amino acids we deal with.  These include:

Branched chain amino acid disorders

• Isovaleric acidemia (leucine): creates a sweaty smell
• Propionic acidemia 
• Maple Syrup Urine Disease

Methionine metabolism disorders (homocysteineuria and related sulfur metabolism problems)

Tyrosine metabolism disorders

• Tyrosinemia type I
• Tyrosinemia type II
• Alkaptonuria

D-glyceric aciduria 

Iminoglycinuria (proline, hydroxyproline, glycine)

Guanidinoacetate methyltransferase deficiency

Hyperprolinemia, type I and II; usually benign

Δ1-Pyrroline-5-carboxylate synthetase deficiency (associated with high ammonia levels)

Prolidase deficiency (proline, hydroxyproline)

Hyperlysinemia (lysine)

Glutaric acidemia type I (carnitine, lysine, tryptophan)

Nonketotic hyperglycinemia (Glycine)

γ-Glutamylcysteine synthetase deficiency (glutathione deficiency); tied in with G6PD

Pyroglutamic aciduria (glutathione); tied in with G6PD

Hyper-β-alaninemia (alanine, taurine, GABA, β-aminoisobutyrate)

Methylmalonate/malonate semialdehyde dehydrogenase issues

Pyridoxine dependency

GABA-transaminase deficiency (GABA, β-alanine, carnosine)

4-Hydroxybutyric aciduria (glycine)

Most of these are non-starters, and I don't see any which are explicitly tied to arginine, but it's worth sifting through them to see what might fit.

Every question spawns a hundred more questions.  This is the boring footwork of diagnosis, but it's essential.  Somewhere out there is the answer, all I have to do is sniff it out.