Hartnup disease

An 11-year-old boy is brought in by his parent to the clinic when she noticed a skin rash over his chest and arms over the past 2 weeks. The boy has noticed that the rash becomes pruritic and painful to touch when he goes outside during recess at school. He also complains of diarrhea over the past several weeks. His parent states his developmental history has been unremarkable except for similar rashes when he was 5 years old that resolved spontaneously. He takes no medications and has not received treatment for any condition in the past. Temperature is 37.0°C (98.6°F), pulse is 72/min, respirations are 18/min, and blood pressure is 115/72 mmHg. BMI is 19 kg/m2. Physical examination reveals well-defined, hyperpigmented, hyperkeratotic, symmetrical, thick scaly plaques surrounded by erythema on the dorsa of the hands, arms, feet, up to the knees, and along the sides of the neck. Urine studies are shown below:

 

Urine
Erythrocytes	0/hpf
Leukocytes	10/hpf
Sediment	None
Urine chromatography	Neutral amino acids

Genetic testing is performed and the diagnosis is confirmed. Which of the following is the most likely pathological mechanism of this patient’s condition?