Differential Diagnosis for Left Lower Back Pain

Single Most Likely Diagnosis

• Musculoskeletal strain: Given the recent history of a treated UTI and the presence of minimal bilateral hydronephrosis without visible stones, musculoskeletal strain is a plausible cause for the left lower back pain, especially if the patient has been experiencing discomfort or altered mobility post-UTI treatment.

Other Likely Diagnoses

• Pyelonephritis or recurrent UTI: Despite a recent treated UTI and normal UA, the possibility of pyelonephritis or a recurrent UTI cannot be entirely ruled out, especially if the initial infection was not fully cleared or if the patient has an underlying condition predisposing to UTIs.
• Nephropathy or interstitial nephritis: This could be a consequence of the recent UTI or its treatment, potentially causing inflammation and leading to back pain.
• Constipation: Although not directly related to the UTI, constipation can cause lower back pain and is a common issue that might coincide with or be exacerbated by the discomfort from a UTI.

Do Not Miss Diagnoses

• Kidney cancer or other renal tumors: Although rare, these conditions can present with back pain and hydronephrosis, making them critical to consider to avoid a delayed diagnosis.
• Aortic aneurysm or dissection: This is a medical emergency that can present with back pain and would be catastrophic if missed.
• Spinal infection (e.g., osteomyelitis, discitis): Infections of the spine can cause severe back pain and are important to diagnose early to prevent serious complications.
• Pulmonary embolism: While less common, pulmonary embolism can cause back pain and is a life-threatening condition that requires immediate attention.

Rare Diagnoses

• Retroperitoneal fibrosis: A rare condition characterized by the proliferation of fibrous tissue in the retroperitoneum, which can cause back pain and hydronephrosis.
• Amyloidosis: A group of diseases resulting from the abnormal deposition of amyloid (a particular type of protein) in various tissues, which can affect the kidneys and cause back pain among other symptoms.
• Sarcoidosis: An autoimmune disease that can affect multiple organs, including the kidneys, and cause a variety of symptoms including back pain.

Each of these diagnoses has a different level of likelihood based on the provided clinical scenario, but considering them is crucial for comprehensive patient care.