Viral Gastroenteritis: This is the most common cause of loose motions in a young adult, often due to norovirus or rotavirus, and typically presents with sudden onset of diarrhea, vomiting, and abdominal cramps.

Bacterial Gastroenteritis: Caused by bacteria such as E. coli, Salmonella, or Campylobacter, often from contaminated food or water, presenting with diarrhea, abdominal pain, and sometimes fever.
Food Poisoning: Similar to bacterial gastroenteritis but typically has a shorter incubation period and is often related to the consumption of specific foods.
Irritable Bowel Syndrome (IBS): A chronic condition characterized by recurring abdominal pain associated with a change in bowel habits, which could include loose motions, but usually has a more gradual onset.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with chronic diarrhea, abdominal pain, and weight loss. Although less common, missing these diagnoses can lead to significant morbidity.
Giardiasis: An intestinal infection caused by the parasite Giardia lamblia, which can lead to severe diarrhea and dehydration if not treated.
Cholera: A diarrheal disease caused by Vibrio cholerae, which can lead to severe dehydration and death if not promptly treated, especially in areas where cholera is common.

Celiac Disease: An autoimmune disorder where the ingestion of gluten leads to damage in the small intestine, causing chronic diarrhea among other symptoms.
Whipple's Disease: A rare, systemic bacterial infection (Tropheryma whipplei) that can cause malabsorption, diarrhea, and weight loss.
Lactose Intolerance: The inability to digest lactose, a sugar found in milk, leading to bloating, gas, and diarrhea after consuming lactose-containing products. While not rare, it's less likely to be considered initially in the context of acute loose motions.