From AGA twitter feed link/Gastroenterology Volume 147, Issue 4, Pages e3–e4, October 2014.:

Question: A 6-year-old Caucasian boy presented with recurrent episodes of fatigue, abdominal pain, and diarrhea. In between these episodes, he had good exercise tolerance. He has never traveled outside Hawaii and Western Europe, had no known allergies, and was not taking any medication. An unexplained splenomegaly had first been discovered 6 months ago.

Clinical examination was unremarkable except for growth below the expected range (FigureFigure A) and splenomegaly with a palpable spleen 4 cm below coastal margin. There was no heart murmur, dyspnea, clubbing, icterus, eczema, or lymphadenopathy. Neurologic development was adequate for age.

Laboratory workup showed no anemia and normal lymphocyte subsets. The thrombocytes were in the low normal range (155 × 10⁹/L) and aspartate transaminase (56 U/L) was mildly elevated. The fecal pancreatic elastase and calprotectin were in the normal range. Ultrasonography revealed enlarged liver and spleen without evidence of portal vein thrombosis or focal lesions. A chest x-ray displayed bilateral interstitial lung disease with a reticulonodular pattern.

What is the most likely diagnosis? 

Here’s the link: Answer and explanation.