A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 15-year-old girl is brought to the physician by her parents because she has not had menstrual bleeding for the past 2 months. Menses had previously occurred at irregular 15–45 day intervals with moderate to heavy flow. Menarche was at the age of 14 years. Eight months ago, she was diagnosed with bipolar disorder and treatment with risperidone was begun. Her parents report that she is very conscious of her weight and appearance. She is 168 cm (5 ft 5 in) tall and weighs 76 kg (168 lb); BMI is 26.9 kg/m2. Pelvic examination shows a normal vagina and cervix. Serum hormone studies show:
Prolactin 14 ng/mL
Follicle-stimulating hormone 5 mIU/mL
Luteinizing hormone 5.2 mIU/mL
Progesterone 0.9 ng/mL (follicular N <3; luteal N >3–5)
Testosterone 2.7 nmol/L (N <3.5)
A urine pregnancy test is negative. Which of the following is the most likely cause of her symptoms?"
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A
Primary ovarian insufficiencyIncorrect. POI would show markedly elevated FSH (>40 mIU/mL); FSH here is normal (5), excluding this diagnosis.
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B
Anovulatory cyclesCorrect. anovulatory cycles are common in the first 2-3 years post-menarche due to immature HPO axis; low progesterone confirms absent ovulation and normal prolactin rules out the medication effect.
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C
Uterine leiomyomasIncorrect. Uterine leiomyomas are rare in adolescents and typically present with heavy menstrual bleeding, not amenorrhea.
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D
Adverse effect of medicationIncorrect. Risperidone can cause hyperprolactinemia and amenorrhea via D2 blockade, but the prolactin is normal (14 ng/mL), specifically ruling this out.
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E
Anorexia nervosaIncorrect. Anorexia nervosa produces functional hypothalamic amenorrhea with low FSH/LH, low estrogen, and low BMI — but this patient has a BMI of 26.9 and only weight preoccupation, not the energy deficit required for hypothalamic amenorrhea.
↑ Tap an answer to reveal the reasoning
Answer: B. A 15-year-old just one year post-menarche with previously irregular cycles (15–45 day intervals), now amenorrheic for 2 months, BMI 26.9 (overweight, not anorexic), normal prolactin, normal FSH/LH (in midcycle range), low progesterone, and borderline testosterone — most likely has anovulatory cycles, which are physiologic in the first 2-3 years post-menarche due to immature HPO axis.
The key labs: prolactin is normal (rules out hyperprolactinemia from risperidone — option D), FSH is not elevated (rules out primary ovarian insufficiency — option A), testosterone is upper-normal (could suggest mild PCOS picture but not full criteria here), and progesterone is low (consistent with no ovulation — luteal phase would have progesterone >3-5). Without a corpus luteum, no progesterone is produced and cycles become irregular.
Adolescent anovulation is common — up to 50% of cycles in the first 2 years after menarche are anovulatory because of immature positive feedback of estrogen on the hypothalamic-pituitary axis (failure to mount the LH surge). This produces unopposed estrogen exposure, irregular bleeding, and sometimes amenorrhea or heavy bleeding.
Option D (risperidone-induced hyperprolactinemia) would be expected to be on the differential, but the NORMAL prolactin specifically rules this out — a great example of the stem giving you the lab to exclude a tempting answer. POI would show elevated FSH (>40). Leiomyomas in a 15-year-old are very unusual and would present with heavy bleeding, not amenorrhea.
Clinical pearl: in adolescents with irregular cycles in the first 2-3 years post-menarche and a benign workup, observation and reassurance are appropriate; persistent anovulation beyond this window or with hyperandrogenic features warrants PCOS workup.
**Why each option:**
**A.** POI would show markedly elevated FSH (>40 mIU/mL); FSH here is normal (5), excluding this diagnosis.
**B.** Correct — anovulatory cycles are common in the first 2-3 years post-menarche due to immature HPO axis; low progesterone confirms absent ovulation and normal prolactin rules out the medication effect.
**C.** Uterine leiomyomas are rare in adolescents and typically present with heavy menstrual bleeding, not amenorrhea.
**D.** Risperidone can cause hyperprolactinemia and amenorrhea via D2 blockade, but the prolactin is normal (14 ng/mL), specifically ruling this out.