USMLE Study Guide 2026

5α-reductase deficiency is a disorder of sex development where testosterone cannot be converted to dihydrotestosterone (DHT). DHT is essential for the development of external male genitalia in utero. Individuals with this XY genetic makeup may present with ambiguous genitalia at birth, often virilizing at puberty due to increased testosterone.

ACE inhibitors prevent bradykinin degradation, causing dry cough in up to 15% of patients; ARBs do not inhibit bradykinin breakdown and lack this side effect.

In approximately 60% of individuals, the SA nodal artery arises from the right coronary artery; in the remaining ~40%, it arises from the left circumflex artery.

Fluoxetine has the longest half-life (~1-4 days) among SSRIs due to its active metabolite norfluoxetine, making discontinuation syndrome rare.

Rathke's pouch is an ectodermal invagination from the roof of the primitive oral cavity that gives rise to the anterior pituitary gland (adenohypophysis). Craniopharyngiomas are tumors that arise from remnants of Rathke's pouch. These tumors are typically suprasellar and can cause symptoms due to compression of surrounding structures like the optic chiasm or hypothalamus, often leading to endocrine dysfunction.

Aminoglycosides cause misreading at the 30S ribosome, incorporating wrong amino acids that create aberrant proteins which disrupt the cell membrane, causing bactericidal cell death.