Splenomegaly due to Splenic Infarction or Rupture: The patient's sudden onset of left-sided abdominal pain, nausea, tenderness in the left flank, moderate guarding, and an enlarged spleen (8cm below the left coastal margin) are highly suggestive of a splenic issue. The history of similar episodes in the past could indicate a chronic condition leading to splenomegaly, with the current episode possibly being an acute infarction or rupture.

Nephrolithiasis (Kidney Stones): The patient's back pain and sudden onset of severe pain could be indicative of a kidney stone, especially if the stone is obstructing the ureter. However, the presence of an enlarged spleen and the specific location of the pain might make this less likely.
Diverticulitis: Although more common on the left side, the absence of changes in bowel habits, fever, or specific tenderness in the left lower quadrant makes this diagnosis less likely. However, it cannot be ruled out without further investigation.
Pancreatitis: The location of the pain and the presence of nausea could suggest pancreatitis, but the lack of specific symptoms like radiation of pain to the back or elevated pancreatic enzymes makes this less likely.

Aortic Dissection or Aneurysm: Although the pain is on the left side, aortic dissection or aneurysm can present with sudden, severe back or abdominal pain and is a life-threatening condition that requires immediate diagnosis and treatment.
Ruptured Abdominal Aortic Aneurysm (AAA): Similar to aortic dissection, a ruptured AAA is a medical emergency that presents with sudden, severe abdominal or back pain and can be fatal if not promptly treated.
Ectopic Pregnancy (in a female patient, but included for completeness): Though the patient is male, in any case of sudden abdominal pain, especially if the diagnosis is not clear, considering an ectopic pregnancy in female patients is crucial due to its high mortality rate if missed.

Wandering Spleen: A rare condition where the spleen is not in its normal position due to the laxity of the splenic ligaments, which can cause intermittent abdominal pain and splenomegaly.
Splenic Artery Aneurysm: A rare condition that can cause abdominal pain if the aneurysm ruptures, though it's more commonly associated with hemorrhage rather than the symptoms described.
Left Upper Quadrant Mass (e.g., pancreatic or renal tumor): Although less likely given the acute presentation, a mass in the left upper quadrant could cause pain and splenomegaly due to compression or invasion of adjacent structures.