Final answer:
The described symptoms and test results suggest the patient is most likely suffering from lactose intolerance, a condition where the small intestine lacks enough lactase to digest lactose, leading to fermentation by bacteria in the large intestine and resulting in various gastrointestinal symptoms.
Step-by-step explanation:
A patient presenting with diarrhea and positive hydrogen breath tests, along with a positive stool test for reducing substances, low stool pH, and increased stool osmotic gap is most likely suffering from lactose intolerance. The hydrogen breath test is particularly indicative of this condition as it shows increased hydrogen levels due to the bacterial fermentation of undigested lactose in the colon. In individuals with lactose intolerance, the small intestine doesn't produce sufficient lactase, the enzyme required to digest lactose, leading to symptoms like gas, abdominal cramps, bloating, and diarrhea.
While an intestinal biopsy can confirm lactose intolerance by measuring lactase enzymes, it is not routinely used due to the need for specialized laboratories and expertise. Instead, the existence of symptoms along with supportive findings from the hydrogen breath test, stool acidity test, and the presence of reducing substances in the stool are often sufficient for diagnosis. The patient's low stool pH reflects the fermentation process of lactose, and the increased stool osmotic gap is consistent with the osmotic diarrhea caused by undigested lactose drawing water into the intestine.