Which of the following is the most common cause of primary amenorrhea in a phenotypically normal female with normal secondary sexual characteristics?

Explanation

## Primary Amenorrhea: Müllerian Agenesis **Key Point:** Müllerian agenesis (Mayer-Rokitansky-Küster-Hauser syndrome, MRKH) is the second most common cause of primary amenorrhea overall, but the **most common cause in phenotypically normal females with normal secondary sexual characteristics**. ### Why Müllerian Agenesis? - **Karyotype:** 46,XX (normal) - **Secondary sexual characteristics:** Normal (breast development, pubic/axillary hair present) - **Ovarian function:** Normal — FSH, LH, and estrogen levels are normal - **Presentation:** Primary amenorrhea with normal pubertal development but absent uterus and upper 2/3 of vagina - **Frequency:** Occurs in ~1 in 4,500 females ### Diagnostic Features | Feature | Müllerian Agenesis | Gonadal Dysgenesis | Androgen Insensitivity | | --- | --- | --- | --- | | Karyotype | 46,XX | 45,X or mosaic | 46,XY | | Breast development | Normal | Absent/sparse | Normal | | Pubic hair | Normal | Absent/sparse | Absent/sparse | | Ovaries | Normal | Streak gonads | Testes (intra-abdominal) | | FSH/LH | Normal | Elevated | Normal/elevated | | Uterus | Absent | Present | Absent | **High-Yield:** The key distinguishing feature is that the patient has **normal secondary sexual characteristics** — this rules out gonadal dysgenesis (Turner syndrome) and androgen insensitivity syndrome (AIS), both of which present with absent or sparse secondary sexual characteristics. **Clinical Pearl:** Diagnosis is confirmed by imaging (ultrasound or MRI) showing absent uterus with normal ovaries and normal hormonal profile (normal FSH, LH, estrogen). **Tip:** When a question states "normal secondary sexual characteristics" in primary amenorrhea, think Müllerian agenesis or imperforate hymen. Imperforate hymen is rarer; MRKH is more common.