A 22 year old man is brought to the ER with abdominal pain. It is severe and he has had several such episodes in the past. The pain is central abdominal pain of a dull aching quality. his mother is with him and says that he has been behaving odd today. He had his last bowel movement today and there was no blood. He denies any rashes or photosensitivity. He says he does not trust his primary care doctor because he keeps sending him to different doctors who have done multiple tests on him and have not come up with any diagnosis.
You perform an extensive examination and besides vague abdominal tenderness are not able to elicit any specific findings.  He is afebrile and normotensive. Rectal exam shows normal stool and guiac test is negative.
He has had a CT scan of his abdomen, Spine and pelvis. He underwent an angiogram for his mesenteric vessels which was normal. His CBC shows WBC count of 15,000, platelets of 168,000 and Hb. 14. His Liver function tests are normal. Serum chemistry shows Na 129, K 4.0, Cl 95, HCO3 23, BUN 18 and Cr. 1.0 & Glucose 103mg%. Amylase and lipase are normal. His EKG shows no ischemia but he has a heart rate of 118.

Q1. What is the correct diagnosis for this patient?

Fill in the blank _______________________________

Answer was acute intermittent porphyria.

Objective is to know when to suspect unusual diseases. In the case of abdominal pain, start suspecting these as soon as imaging and stool guiac test are normal.
In this case with abdominal pain, there are a few possibilities. One must take into account that the patient has an altered mental status inspite of almost normal electrolytes. He also has hyponatremia. Then there is tachycardia and normal BP and normal Glucose and creatinine.
The differential includes
Addison's disease, DKA, Black widow spider bite, Abdominal epilepsy, mesenteric ischemia and Acute intermittent porphyria.
Let us tease out each one and see if it fits into this patient's picture.

Differential Diagnosis Features of the differential diagnosis present   in this case Features of the differential diagnosis absent in this case Likelihood of the diagnosis in this case
Addison's crisis Abdominal pain, hyponatremia Hyperkalemia, hypotension, Metabolic acidosis, elevated BUN Fair
DKA (diabetic Keto-acidosis) Young patient, abdominal pain Low HCO3, hyperglycemia, no resolution without treatment Unlikely
Black widow spider bite Abdominal pain Not recurrent, Mental status normal, rigid abdomen Unlikely
Hereditary angio-edema (C1 esterase deficiency) Abdominal pain Rash, hereditary, family history Fair
Abdominal epilepsy Abdominal pain Rare, abdominal rigidity Unlikely
Irritable bowel syndrome Almost normal labs & imaging Tachycardia, altered mental status, hyponatremia  
Acute intermittent porphyria Abdominal pain, hyponatremia, altered mental status, normal routine labs, Tachycardia   High
Fulfils the major problems in the case and is the diagnosis