Differential Diagnosis for the Patient's Symptoms

The patient presents with severe lower abdominal pain, suprapubic fullness, and tenderness to palpation of the lower abdomen. Given these symptoms and the ultrasound image of the abdomen, the following differential diagnoses can be considered:

• Single Most Likely Diagnosis

  • Urinary Retention: The patient's symptoms of suprapubic fullness and tenderness, along with the recent use of diphenhydramine (an anticholinergic medication that can cause urinary retention), make this the most likely diagnosis. The normal sphincter tone and prostate gland on rectal examination do not rule out urinary retention, especially if it is due to medication side effects.

• Other Likely Diagnoses

  • Nephrolithiasis (E): Although the patient's pain is in the lower abdomen, kidney stones can sometimes present with pain in this area, especially if the stone is moving down the ureter. However, the absence of flank pain or radiating pain to the groin makes this less likely.
  • Bowel Obstruction (C): This could cause severe abdominal pain and tenderness, but the presence of suprapubic fullness and the patient's recent medication history point more towards a urinary issue.

• Do Not Miss Diagnoses

  • Abdominal Aortic Aneurysm (A): Although the patient's symptoms do not classically present as an abdominal aortic aneurysm (which typically includes a pulsatile abdominal mass and back pain), this is a potentially life-threatening condition that should always be considered in older adults with severe abdominal pain.
  • Benign Prostatic Hyperplasia (B): While the rectal examination was normal, prostatic enlargement could contribute to urinary retention, especially in the context of anticholinergic medication use.

• Rare Diagnoses

  • Medication Adverse Effect (D) other than urinary retention: While diphenhydramine is known to cause urinary retention, other medications or less common side effects could potentially cause abdominal pain, though this would be less likely given the patient's current medication list.
  • Other rare causes of abdominal pain: Such as intra-abdominal shunt malfunction (if the patient had a history of such a procedure), or less common infections like diverticulitis, though these would not be as directly linked to the patient's presentation and history as the other options.