Musculoskeletal Strain: This is the most common cause of lower back pain, often resulting from overuse, heavy lifting, or sudden movement. The pain is typically localized to the lower back and may radiate to the buttocks or thighs.

Lumbar Disc Herniation: Herniation of the lumbar discs can cause pain in the lower back, which may radiate to the legs due to nerve compression. Symptoms can include numbness, tingling, and weakness in the legs.
Kidney Stones: While kidney stones can cause severe, sharp pain, they often present with flank pain that may radiate to the groin. The pain can be accompanied by nausea, vomiting, and urinary frequency.
Urinary Tract Infection (UTI): A UTI can cause lower back pain, especially if the infection has reached the kidneys (pyelonephritis). Symptoms include fever, dysuria, and frequent urination.
Degenerative Disc Disease: This condition involves wear and tear on the spinal discs, leading to chronic lower back pain. It's more common in older adults and can be exacerbated by activity.

Abdominal Aortic Aneurysm: Although less common, an abdominal aortic aneurysm can present with lower back pain, especially if it's leaking or ruptured. This is a medical emergency requiring immediate attention.
Pulmonary Embolism: In rare cases, a pulmonary embolism can cause referred pain to the lower back. This condition is life-threatening and requires prompt diagnosis and treatment.
Osteomyelitis or Discitis: Infections of the bone or disc can cause severe back pain and are medical emergencies, especially in immunocompromised patients or those with a history of intravenous drug use.

Spinal Tumors: Both primary and metastatic tumors can cause lower back pain, often accompanied by other symptoms like weight loss, night pain, or neurological deficits.
Inflammatory Spondyloarthropathies: Conditions like ankylosing spondylitis can cause chronic lower back pain and stiffness, particularly in younger patients.
Referred Pain from Intra-Abdominal Pathology: Certain conditions like pancreatitis or a ruptured ovarian cyst can cause referred pain to the lower back. These conditions often have additional symptoms that can guide the diagnosis.