Musculoskeletal strain: Given the absence of specific urinary or gastrointestinal symptoms, musculoskeletal strain is a common cause of flank pain that can occur due to overuse, injury, or poor posture.

Kidney stone: Although there are no pus cells in the urine, kidney stones can cause severe flank pain and do not always present with urinary tract infection symptoms.
Pyelonephritis without significant bacteriuria: Early in the course of the disease or in cases of infection with certain organisms, the urine may not show pus cells.
Diverticulitis: Inflammation of a diverticulum in the sigmoid colon can cause left flank pain, especially if the diverticulum is located near the left kidney.

Appendicitis (if appendix is retrocecal): Although appendicitis typically presents with right lower quadrant pain, a retrocecal appendix can cause pain that is perceived in the flank.
Ectopic pregnancy: In women of childbearing age, ectopic pregnancy is a life-threatening condition that can cause flank pain.
Aortic dissection or aneurysm: These are vascular emergencies that can present with sudden, severe flank or back pain.
Pneumonia (especially lower lobe): In some cases, pneumonia can cause referred pain to the flank area.

Pancreatitis: Inflammation of the pancreas can cause severe abdominal pain that may radiate to the flank.
Retroperitoneal fibrosis: A rare condition characterized by the proliferation of fibrous tissue in the retroperitoneum, which can cause flank pain and obstructive uropathy.
Tumor (renal or adrenal): Although less common, tumors in the kidney or adrenal gland can cause flank pain due to compression or invasion of surrounding structures.