A 41-year-old woman is evaluated in the emergency department for severe, diffuse abdominal… — USMLE Step 3

A 41-year-old woman is evaluated in the emergency department for severe, diffuse abdominal pain that started two days ago. The pain is continuous, non-radiating, and accompanied by persistent nausea, vomiting, and new-onset generalized weakness. One week ago, she was diagnosed with an uncomplicated lower urinary tract infection and began a course of trimethoprim-sulfamethoxazole. Her past medical history is notable for generalized anxiety disorder. On presentation, her temperature is $37.1^{\circ}\text{C}$ ( $98.8^{\circ}\text{F}$ ), blood pressure is $168/102 \text{ mmHg}$ , heart rate is $118/\text{min}$ , and respiratory rate is $18/\text{min}$ . Physical examination reveals a distressed, diaphoretic woman. Her abdomen is diffusely tender to deep palpation, but there is no guarding, rigidity, or rebound tenderness. Neurologic examination demonstrates $4/5$ strength in the proximal upper and lower extremities bilaterally, with diminished deep tendon reflexes. Laboratory studies show a leukocyte count of $8,500/\mu\text{L}$ , hemoglobin of $13.8 \text{ g/dL}$ , and a serum sodium of $128 \text{ mEq/L}$ . Liver function tests and serum lipase are within normal limits. A non-contrast CT scan of the abdomen shows no abnormalities. Urinalysis reveals no red or white blood cells, but the urine appears dark reddish-brown after standing. Which of the following is the most likely diagnosis?

Acute intermittent porphyriaCevap
B
Acute mesenteric ischemia
C
Guillain-Barré syndrome
D
Lead toxicity
E
Functional neurological symptom disorder

Cevap

Acute intermittent porphyria

The correct diagnosis is Acute Intermittent Porphyria (AIP). This patient presents with a classic triad of symptoms for an acute porphyric attack: severe abdominal pain (often without peritoneal signs), autonomic instability (tachycardia, hypertension, diaphoresis), and neuropsychiatric/neuromuscular manifestations (proximal weakness, diminished deep tendon reflexes). The attack in this vignette was precipitated by the administration of trimethoprim-sulfamethoxazole, a classic porphyrinogenic drug that induces cytochrome P450 enzymes. Additional key features confirming the diagnosis include hyponatremia (commonly due to the syndrome of inappropriate antidiuretic hormone secretion, SIADH, associated with AIP) and dark reddish-brown urine, which occurs when excess porphobilinogen oxidizes to porphobilin upon standing in ambient light. Normal abdominal imaging and normal lipase effectively rule out primary gastrointestinal emergencies.

Adım Adım Çözüm

Analyze the clinical presentation and medication history.

Identified a triad of severe neuropathic abdominal pain, autonomic hyperactivity (hypertension, tachycardia, diaphoresis), and neuromuscular weakness (proximal deficits, areflexia) precipitated by a sulfonamide antibiotic.

Establishing the core clinical syndrome and potential triggers is the first step in formulating a targeted differential diagnosis.

Evaluate the objective diagnostic data.

Noted normal abdominal CT, normal lipase, and normal hemoglobin, which safely rule out structural GI emergencies and anemia-associated toxicities. Identified significant hyponatremia and dark urine lacking erythrocytes.

Diagnostic data must be synthesized to eliminate high-morbidity structural conditions and highlight subtle metabolic derangements.

Synthesize the findings to isolate the most likely etiology.

Concluded that Acute Intermittent Porphyria uniquely unifies the multi-system findings (GI, neuro, autonomic, renal/electrolytes), whereas other diagnoses fail to account for the entire clinical picture.

A comprehensive differential diagnosis must account for all of the patient's objective, atypical findings without relying on premature closure.

Anahtar Kavram

Synthesizing multi-system clinical findings (abdominal pain, neuropathy, autonomic instability, hyponatremia) and medication history to differentiate Acute Intermittent Porphyria from overlapping gastrointestinal and neurological conditions.

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