United States Medical Licensing Step 1: USMLE-STEP-1

Correct Answer: A

Section: Biochemistry PBG deaminase (also called hydroxymethylbilane synthase) catalyzes the heme biosynthesis reaction involving the head-to-tail condensation of four molecules of PBG to produce the linear tetrapyrrole intermediate, hydroxymethylbilane. Hydroxymethylbilane can nonenzymatically cyclize into uroporphyrinogen I, which is why PBG deaminase is also known as uroporphyrinogen I synthase. ALA is the precursor for PBG, thus a defect in PBG deaminase would lead to excess ALA excretion. The compounds in choices BE all represent products of reactions that are downstream of PBG deaminase in the heme biosynthetic pathway and thus would not be excreted in high amounts in someone with AIP.

Correct Answer: C

Section: Pharmacology Edema and hypertension will occur with any corticosteroid that has mineralocoid properties if given in high dosage over a long period. Prednisone, though much less potent than cortisol as a mineralocorticoid, retains some salt-retaining potency. Newer glucocorticoids, such as dexamethasone and triamcinolone, are much less likely to retain salt and water. Buffalo hump (choice A) and moon facies (choice D) reflect the fat metabolism effects of glucocorticoid activity. Easy bruising (choice B) and striae reflect the catabolic action of glucocorticoids on proteins. The mechanism of central nervous system dysfunction (choice E) is not understood, but it is associated with glucocorticoid, not mineralocorticoid, activity.

Correct Answer: C