Q-Bank Breakdown: Turner syndrome — Why Every Answer Choice Matters

You just missed a question on Turner syndrome and thought, “Okay, it’s monosomy X—moving on.” Not so fast. On Step exams, the real points live in the answer choices you didn’t pick. Turner syndrome vignettes are classic, but the distractors are where test writers hide the traps: androgen insensitivity, Fragile X, congenital adrenal hyperplasia, trisomies, and more. Let’s break down a Turner-style Q-bank question the way you should review it—why the right answer is right, and why every other option is wrong.

Tag: Genetics > Chromosomal Disorders & Syndromes

---

The Clinical Vignette (Turner Syndrome)

A 15-year-old phenotypic female is brought in for evaluation of primary amenorrhea. She is short with a webbed neck and widely spaced nipples. Physical exam shows Tanner stage I breast development. Pelvic ultrasound shows streak ovaries. Labs show elevated FSH and LH.

Question: What is the most likely karyotype?

---

Correct Answer: 45,X (Turner Syndrome)

Why it’s Turner syndrome

This vignette is loaded with Turner clues:

• Short stature
• Webbed neck (cystic hygroma remnant)
• Shield chest / widely spaced nipples
• Streak ovaries → gonadal dysgenesis
• Primary amenorrhea
• Low estrogen → poor breast development
• High FSH/LH → hypergonadotropic hypogonadism (primary ovarian failure)

Pathophysiology you should know

• Monosomy X (45,X) leads to ovarian follicle loss early → streak gonads → low estrogen.
• Lack of estrogen removes negative feedback → ↑ FSH and ↑ LH.

💡

High-yield association: Turner is a chromosomal disorder with gonadal dysgenesis causing primary ovarian failure.

---

High-Yield Turner Syndrome Facts (USMLE Core)

Key clinical features

• Short stature
• Webbed neck, low posterior hairline
• Shield chest with widely spaced nipples
• Lymphedema of hands/feet (especially in infancy)
• Streak ovaries → infertility
• Primary amenorrhea

Major associated anomalies (testable!)

Genetics/diagnostics pearls

• Classic karyotype: 45,X
• Many patients are mosaic (e.g., 45,X/46,XX) → variable phenotype.
• No Barr body in pure 45,X (because no “extra” X to inactivate).
• Screening/confirming: karyotype (peripheral blood), consider mosaicism if phenotype suggests Turner but initial karyotype is normal.

---

Now the Money: Why the Distractors Are Wrong

Below are common answer choices that show up in the same question—and how to kill them fast.

---

Distractor 1: 46,XX (Normal female)

Why it’s tempting: Patient is phenotypically female.

Why it’s wrong:

• Normal karyotype shouldn’t produce:
  • streak ovaries
  • hypergonadotropic hypogonadism
  • classic Turner body habitus (webbed neck, shield chest)
• Primary amenorrhea in a 46,XX patient would push you toward:
  • Müllerian agenesis (normal secondary sex characteristics)
  • primary ovarian insufficiency (less “syndromic” body findings)

---

Distractor 2: 46,XY (Androgen Insensitivity Syndrome)

Classic AIS vignette: Phenotypic female with primary amenorrhea, normal breast development, scant/absent pubic hair, undescended testes, no uterus.

Why it’s wrong here:

• Turner: no breast development (low estrogen)
• AIS: breast development present (testicular androgens aromatize → estrogen)
• Turner: streak ovaries
• AIS: testes, often intra-abdominal/inguinal
• AIS has low/normal FSH/LH relative to Turner’s strongly elevated gonadotropins (physiology varies, but Turner’s hypergonadotropic pattern is the board-style cue)

Fast rule:

• Primary amenorrhea + breasts present + little pubic hair → think AIS (46,XY).
• Primary amenorrhea + no breasts + streak ovaries + short/webbed neck → Turner (45,X).

---

Distractor 3: 47,XXX (Triple X)

Typical features:

• Often asymptomatic
• Sometimes tall stature, learning difficulties
• Usually normal puberty and fertility

Why it’s wrong:

• Turner is short, Triple X tends to be tall (if anything).
• Triple X does not classically cause streak gonads or the Turner congenital anomalies.
• Barr bodies: Triple X would have 2 Barr bodies (all extra X beyond one get inactivated).

---

Distractor 4: 47,XXY (Klinefelter Syndrome)

Classic Klinefelter vignette: Tall male, small firm testes, infertility, gynecomastia, ↓ testosterone, ↑ LH/FSH.

Why it’s wrong:

• Patient is phenotypically female with Turner stigmata.
• Klinefelter presents with male phenotype and testicular failure, not streak ovaries.
• Klinefelter: tall, Turner: short.

Exam tip: If the stem screams “primary hypogonadism,” don’t stop there—match the phenotype:

• Male + tall + small testes → Klinefelter (47,XXY)
• Female + short + webbed neck → Turner (45,X)

---

Distractor 5: Fragile X Syndrome (CGG repeat on X chromosome)

Classic features:

• Intellectual disability, autism
• Long face, large ears, macroorchidism (post-puberty)
• Anticipation (worsens in successive generations)

Why it’s wrong:

• Fragile X is a single-gene repeat expansion, not a classic “streak gonads + congenital anomalies” syndrome.
• Primary amenorrhea + Turner body features points to chromosomal aneuploidy, not Fragile X.

Board trap: Fragile X can be associated with premature ovarian insufficiency in some carriers, but that presentation won’t give you webbed neck/coarctation/horseshoe kidney. The stem’s dysmorphology matters.

---

Distractor 6: Congenital Adrenal Hyperplasia (e.g., 21-hydroxylase deficiency)

Classic features (46,XX virilization):

• Ambiguous genitalia at birth
• Early puberty, androgen excess
• Low aldosterone → hypotension, hyperkalemia (salt-wasting form)
• High 17-hydroxyprogesterone

Why it’s wrong:

• Turner is estrogen deficiency and streak ovaries—not androgen excess.
• CAH doesn’t cause a webbed neck or shield chest.
• Amenorrhea in CAH is due to androgen excess disrupting ovulation—not hypergonadotropic hypogonadism.

---

Distractor 7: Trisomy 21 (Down syndrome)

Classic features:

• Upslanting palpebral fissures, single palmar crease, hypotonia
• Congenital heart disease (AV septal defect)
• Duodenal atresia, Hirschsprung disease
• Increased risk of leukemia and early Alzheimer

Why it’s wrong:

• Not a match for the gonadal dysgenesis + short stature + webbed neck package.
• Down syndrome is not classically a primary amenorrhea/streak gonad question.

---

The One-Liner You Should Remember

Turner syndrome = 45,X → streak ovaries → ↓ estrogen → ↑ FSH/LH + short stature + webbed neck + coarctation + horseshoe kidney.

If the stem gives you primary amenorrhea + no secondary sexual characteristics + Turner body habitus, the karyotype answer is practically a giveaway. Your job is not just to pick 45,X—it’s to be able to defend it against every distractor in 10 seconds.

---

Rapid-Fire Comparison Table (High Yield)