Differential Diagnosis for Intermittent, Non-Radiating Flank Pain

Given the symptoms of intermittent, non-radiating flank pain, urinary frequency, tenderness on palpation, a negative pregnancy test, and a negative urinalysis, the following differential diagnoses can be considered:

• Single Most Likely Diagnosis
  • Musculoskeletal Back Pain: This is often the most common cause of flank pain and can be due to strain, overuse, or poor posture. The absence of radiating pain and negative urinalysis makes this a plausible diagnosis.
• Other Likely Diagnoses
  • Urinary Tract Infection (UTI): Despite a negative urinalysis, some UTIs may not show positive results immediately. Symptoms like urinary frequency and flank pain are consistent with a UTI.
  • Kidney Stones (Nephrolithiasis): Although the pain is described as non-radiating, small stones might not cause the typical radiating pain to the groin. A negative urinalysis does not rule out kidney stones, as not all stones cause hematuria.
  • Pyelonephritis: An infection of the kidney, which could present with flank pain and urinary frequency. The negative urinalysis might be a false negative, or the infection could be in its early stages.
• Do Not Miss Diagnoses
  • Ectopic Pregnancy: Although the pregnancy test is negative, it's crucial to consider ectopic pregnancy, especially if the patient is of childbearing age and the test was done early. False negatives can occur.
  • Appendicitis: In rare cases, appendicitis can present with flank pain if the appendix is retrocecal. This diagnosis is critical to consider due to the potential for severe complications if missed.
  • Renal Infarction: A rare but serious condition where the kidney loses blood flow. It can present with acute flank pain and might not always have a clear cause on urinalysis.
• Rare Diagnoses
  • Renal Cell Carcinoma: Although rare, it can cause flank pain. Typically, other symptoms like hematuria or a palpable mass might be present, but early stages could be asymptomatic aside from pain.
  • Polycystic Kidney Disease (PKD): A genetic disorder that can cause cysts to form on the kidneys, leading to flank pain. It's less likely without a family history or other symptoms like hematuria.
  • Papillary Necrosis: A condition where part of the kidney tissue dies due to lack of blood flow. It can cause flank pain and is more common in diabetics or those with analgesic abuse.

Each of these diagnoses should be considered in the context of the patient's full medical history, physical examination, and potentially further diagnostic testing to determine the underlying cause of their symptoms.