Ruptured Appendicitis: This is the most likely diagnosis given the patient's symptoms of intermittent abdominal pain, particularly in the upper epigastric region initially, which later became generalized, along with the ultrasound findings that led to the diagnosis of ruptured appendicitis. The patient's history of abdominal pain, fever (indicated by malaise and signs of infection), and the progression of symptoms support this diagnosis.

Urinary Tract Infection (UTI) or Prostatitis: The patient's symptoms of urinary straining, hesitancy, decreased urine output, and the need to urinate after defecating suggest a possible UTI or prostatitis, especially given the patient's male gender and age. These conditions could coexist with or be separate from the primary diagnosis of ruptured appendicitis.
Gastroenteritis: The episodes of diarrhea could indicate a gastrointestinal infection. However, the presence of abdominal pain and other systemic symptoms might suggest a more complex condition than a simple gastroenteritis.
Diverticulitis: Although less common in this age group, diverticulitis could present with abdominal pain, changes in bowel habits, and possibly urinary symptoms if the inflammation involves adjacent structures.

Perforated Viseral Organ: Conditions like a perforated ulcer or a perforated intestine could present with severe abdominal pain and signs of peritonitis, similar to ruptured appendicitis. Missing these diagnoses could lead to severe consequences, including sepsis and death.
Ectopic Pregnancy (Ruled Out Due to Gender): Although the patient is male, it's essential to consider ectopic pregnancy in females presenting with similar symptoms. In this case, it's not applicable but is a critical "do not miss" diagnosis in the appropriate demographic.
Testicular Torsion: While the patient's symptoms do not strongly suggest testicular torsion, it is a condition that requires immediate attention and could potentially present with abdominal pain if the testicular pain radiates.

Intestinal Obstruction: This could present with abdominal pain, changes in bowel habits, and possibly urinary symptoms if the obstruction affects bowel habits significantly. However, it is less likely given the patient's presentation and the absence of clear signs of obstruction.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis could present with chronic or intermittent abdominal pain, diarrhea, and systemic symptoms. While possible, these are less likely given the acute presentation and the absence of a long-standing history of gastrointestinal symptoms.
Abdominal Aortic Aneurysm: Although rare in this age group, an abdominal aortic aneurysm could present with abdominal pain and is a critical diagnosis not to miss due to its high mortality rate if ruptured.