Understanding sexual differentiation is crucial for students preparing for the USMLE Step 1. This guide covers the key aspects of genetic and hormonal influences on the development of internal and external genitalia, with visual aids, tables, mnemonics, and points to remember.

| Pathway | Hormones/Factors | Outcomes |
|---|---|---|
| XY Chromosome (SRY gene) | Testosterone, MIF | Male internal and external genitalia |
| XX Chromosome (No SRY gene) | Lack of MIF, DHT | Female internal and external genitalia |
| 5α-Reductase Deficiency | Testosterone only | Male internal, ambiguous external genitalia |
| Absence of Sertoli Cells/MIF | Testosterone present | Both male and female internal genitalia |
| Genital Structure | Male Development | Female Development |
|---|---|---|
| Mesonephric (Wolffian) Duct | Seminal vesicles, epididymis, ejaculatory duct, ductus deferens | Degenerates |
| Paramesonephric (Müllerian) Duct | Degenerates due to MIF | Fallopian tubes, uterus, proximal vagina |
| External Genitalia | Penis, scrotum | Clitoris, labia, distal vagina |
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