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Hartnup disease
Characterized by a decreased ability to reabsorb non-polar amino acids (notably tryptophan) in the PCT.
Why does the USMLE give a fuck?
Because tryptophan is a precursor of both serotonin and niacin, so urinary loss of tryptophan due to Hartnup disease or increased utilization of it to make serotonin in carcinoid syndrome leads to niacin deficiency.
| Hartnup disease → ↓ tryptophan → ↓ niacin (B3) → pellagra |
| ↑ serotonin production (carcinoid syndrome) → ↓ tryptophan → ↓ niacin → pellagra |
You might get a vignette of pellagra (dementia, dermatitis, diarrhea) where the answer is “impaired tubular reabsorption of dietary amino acid.”
Fanconi syndrome
Tryptophan is a neutral amino acid. So if they tell you that a patient has tryptophan and charged amino acids in the urine (e.g., proline, hydroxyproline, arginine), think Fanconi syndrome instead of Hartnup disease.
Fanconi syndrome is characterized by a more non-selective inability to reabsorb amino acids from the urine (as well as glucose, phosphate and bicarbonate).
The USMLE will give you a chart with “decreased” or “unchanged” combinations with respect to PCT reabsorption of various solutes. For Fanconi syndrome, you need to know for amino acids, glucose, bicarb, and phosphate, reabsorption is decreased for all of them.
| Fanconi syndrome | Amino acids | Glucose | Bicarb | Phosphate |
| PCT reabsorption (↓ or unchanged) | ↓ | ↓ | ↓ | ↓ |
Because there’s ↓ bicarb reabsorption in Fanconi syndrome, it is a known cause of renal tubular acidosis type II (RTA II).
| Fanconi syndrome can be caused by consumption of expired tetracyclines. |
Don’t confuse Fanconi syndrome with Fanconi anemia. The latter is a congenital aplastic anemia, where the patient also has absent or hypoplastic thumbs or radii.
Then when you go off thinking you’re cool that you know Fanconi syndrome has the latter detail, Diamond-Blackfan anemia is a pure RBC aplasia characterized by triphalangeal thumbs, not absent/hypoplastic ones as in Fanconi syndrome. UWorld for Step 3 actually has a question comparing these two (random I know).
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