Differential Diagnosis for Hypophosphatemia

• Single most likely diagnosis
  • A. Ferric carboxymaltose: This is the most likely cause of hypophosphatemia in this patient. Ferric carboxymaltose, an intravenous iron formulation, is known to cause hypophosphatemia as a side effect. The mechanism involves the inhibition of renal tubular phosphate reabsorption, leading to increased fractional excretion of phosphate, which is consistent with the patient's laboratory results showing a fractional excretion of phosphorus of 21%.
• Other Likely diagnoses
  • D. Prednisone: Although less likely than ferric carboxymaltose, prednisone (a corticosteroid) can also contribute to hypophosphatemia. Corticosteroids can increase renal phosphate excretion, potentially leading to hypophosphatemia, especially with long-term use.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • C. Metabolic acidosis: While the patient's bicarbonate level is slightly low, which might suggest a mild metabolic acidosis, this condition can lead to hypophosphatemia. However, the primary cause of metabolic acidosis would need to be addressed, and in this context, it seems less directly related to the patient's current presentation compared to the effect of ferric carboxymaltose.
• Rare diagnoses
  • B. Low phosphorus diet: Although a diet very low in phosphorus could theoretically lead to hypophosphatemia, this would be less common, especially given the patient's report of a good appetite and the absence of other indications of severe dietary deficiencies. Other rare causes might include certain genetic disorders or other medications not listed, but these would be less likely given the provided information.