A 28-year-old woman presents with acute abdominal pain, tachycardia, and neuropsychiatric symptoms following exposure to a sulfonamide drug. Urine porphobilinogen (PBG) is markedly elevated. A 35-year-old man with a lifelong history of photosensitive blistering lesions on sun-exposed skin and hyperpigmentation presents with normal abdominal function and no acute neurological symptoms. Urine uroporphyrin is elevated. Which clinical feature best distinguishes acute intermittent porphyria from porphyria cutanea tarda?

Explanation

## Distinguishing Acute Intermittent Porphyria from Porphyria Cutanea Tarda ### Clinical Presentation Comparison | Feature | Acute Intermittent Porphyria (AIP) | Porphyria Cutanea Tarda (PCT) | |---------|-------------------------------------|------------------------------| | **Acute neurovisceral crises** | Yes — hallmark feature | No — skin manifestations only | | **Photosensitivity** | Absent | Prominent — blistering, erosions | | **Abdominal/neuropsych symptoms** | Severe, life-threatening | Absent | | **Urine PBG & ALA** | Markedly elevated during acute attack | Normal or mildly elevated | | **Urine uroporphyrin** | Mildly elevated | Markedly elevated | | **Enzyme defect** | PBGD (porphobilinogen deaminase) | UROD (uroporphyrinogen decarboxylase) | | **Triggers** | Drugs, fasting, stress, menses | Alcohol, estrogens, hepatitis C, HIV | ### Key Point: **The defining discriminator between AIP and PCT is the presence of acute neurovisceral crises in AIP.** AIP is an acute hepatic porphyria characterized by life-threatening attacks of abdominal pain, autonomic dysfunction, and neuropsychiatric manifestations. PCT is a cutaneous porphyria with chronic photosensitive skin lesions but NO acute systemic attacks. ### High-Yield: - **AIP = Acute + Neuro** (remember: "A-I-P" = Acute Intermittent Porphyria) - **PCT = Photosensitivity + Chronic** (cutaneous only) - AIP patients require strict avoidance of triggering drugs (barbiturates, sulfonamides, OCPs); PCT patients do not have this constraint. ### Clinical Pearl: A patient with AIP in acute crisis presents with the "surgical abdomen" but normal imaging — this diagnostic confusion is common. The markedly elevated urine PBG during attack (>10× normal) is the biochemical hallmark. In contrast, PCT presents insidiously with fragile skin, hyperpigmentation, and hypertrichosis on dorsal hands — no acute crises. ### Mnemonic: **ACUTE PORPHYRIA = AIP (Abdominal, Acute, Neuro)** **CUTANEOUS PORPHYRIA = PCT (Photosensitive, Chronic, Tarda)**