Differential Diagnosis

The patient's presentation of profound weakness, easy fatigability, anorexia, weight loss, diarrhea, and laboratory findings of hyponatremia, hyperkalemia, and acidosis, along with skin hyperpigmentation, suggests an endocrine disorder. Given her background of AIDS and disseminated histoplasmosis, the differential diagnosis can be categorized as follows:

• Single most likely diagnosis
  • Adrenal insufficiency: The combination of weakness, weight loss, anorexia, diarrhea, hyponatremia, hyperkalemia, and skin hyperpigmentation is highly suggestive of adrenal insufficiency. In a patient with AIDS and disseminated histoplasmosis, adrenal involvement by the infection is a well-documented cause of adrenal insufficiency.
• Other Likely diagnoses
  • Hyporeninemic hypoaldosteronism: This condition could explain the hyperkalemia but would not fully account for the other symptoms and signs, such as skin hyperpigmentation.
  • Renal tubular acidosis: Could be a consideration given the acidosis and electrolyte imbalances, but it doesn't fully explain the clinical picture, especially the skin hyperpigmentation.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Acute adrenal crisis: Although the patient's condition seems chronic, an acute crisis can be precipitated by infection, trauma, or other stresses, and is life-threatening if not promptly treated.
  • Pheochromocytoma: Though less likely given the clinical context, it could cause some of the symptoms and would be critical to diagnose due to its potential for severe, life-threatening complications.
• Rare diagnoses
  • Congenital adrenal hyperplasia: Unlikely in a 45-year-old but could be considered if other diagnoses are ruled out and there's a suggestive family history or specific pattern of hormone abnormalities.
  • Familial glucocorticoid deficiency: A rare genetic disorder that could present similarly to adrenal insufficiency but would be unusual in an adult without a known family history.

Each of these diagnoses should be considered in the context of the patient's immune-compromised state and the potential for opportunistic infections or unusual presentations of common diseases.