Kidney Stone (Nephrolithiasis): This condition is a common cause of severe, episodic pain that radiates from the loin to the groin, often accompanied by decreased urine output due to obstruction. The pain is typically colicky in nature and can be associated with nausea, vomiting, and hematuria.

Pyelonephritis: An infection of the kidney that can cause flank pain, fever, and decreased urine output. It's more common in women and can be a complication of untreated urinary tract infections.
Urinary Retention: Can cause severe pain and decreased urine output. It's more common in men, especially those with benign prostatic hyperplasia (BPH).
Renal Infarction: Although less common, it can present with acute flank pain and decreased urine output, often in the context of atrial fibrillation or other thromboembolic conditions.

Abdominal Aortic Aneurysm (AAA): Although AAA typically presents with abdominal pain, it can radiate to the back and groin. Rupture of an AAA is a medical emergency that can lead to severe pain and decreased urine output due to retroperitoneal bleeding.
Ectopic Pregnancy: In women of childbearing age, ectopic pregnancy can cause severe abdominal and pelvic pain, which might radiate to the groin, and can lead to decreased urine output if there's significant internal bleeding.
Sepsis from a Urinary Source: Sepsis can arise from an untreated urinary tract infection and can present with systemic symptoms including decreased urine output.

Renal Cell Carcinoma: Can cause flank pain and decreased urine output, especially if the tumor obstructs the renal pelvis or invades the renal parenchyma.
Retroperitoneal Fibrosis: A rare condition that can cause obstructive uropathy leading to decreased urine output and flank pain due to encasement of the ureters by fibrotic tissue.
Lymphoma: Can cause renal impairment and pain if the kidneys are involved, either directly by tumor infiltration or indirectly by obstructing the urinary tract.