Differential Diagnosis

The patient presents with progressive abdominal pain, constipation, nausea, chronic cough, significant weight loss, and fatigue. Laboratory studies reveal elevated serum calcium and low serum phosphorus. Based on these findings, the differential diagnosis can be categorized as follows:

• Single most likely diagnosis
  • Squamous cell lung cancer: The patient's 30-year history of tobacco use, chronic cough, and significant weight loss are highly suggestive of lung cancer. Elevated serum calcium (hypercalcemia) is a common paraneoplastic syndrome associated with squamous cell lung cancer, which can produce parathyroid hormone-related protein (PTHrP) leading to hypercalcemia.
• Other Likely diagnoses
  • Multiple myeloma: This is a plasma cell disorder that can cause hypercalcemia due to bone destruction and release of calcium into the bloodstream. Although the patient's serum protein and albumin levels are within normal limits, multiple myeloma should be considered, especially given the patient's age and symptoms of weight loss and fatigue.
  • Primary hyperparathyroidism: Although the patient's parathyroid hormone (PTH) level is at the lower end of the normal range, primary hyperparathyroidism can cause hypercalcemia. However, the presence of other symptoms such as chronic cough and significant weight loss makes this diagnosis less likely.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Pneumonia or lung abscess: The patient's decreased breath sounds in the right middle lobe and chronic cough could be indicative of a pulmonary infection, which would require prompt antibiotic treatment.
  • Malignancy with metastasis: The patient's significant weight loss and abdominal pain could be indicative of a malignancy with metastasis to the abdomen, which would require prompt evaluation and treatment.
• Rare diagnoses
  • Familial hypocalciuric hypercalcemia (FHH): This is a rare genetic disorder that can cause hypercalcemia, but it is less likely given the patient's age and symptoms.
  • Vitamin D intoxication: This is a rare cause of hypercalcemia, but it is less likely given the patient's lack of symptoms suggestive of vitamin D toxicity.