Embryology: twinning: practice question

A Anatomy question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Anatomy · Embryology: twinning

Sample item

Ultrasound of a twin pregnancy shows one placenta with a thin membrane dividing two amniotic sacs and no lambda sign. At what stage did the embryo most likely divide?

AWithin the first 3 days, at the morula stageBBetween day 8 and day 12, at the bilaminar disc stageCBetween day 4 and day 8, at the blastocyst stageDAfter day 13, at the primitive streak stage

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Between day 4 and day 8, at the blastocyst stage

Splitting of the inner cell mass at days 4 to 8 gives monochorionic diamniotic twins, the most common monozygotic type. Splitting before day 4 produces dichorionic diamniotic twins, splitting at days 8 to 12 gives monochorionic monoamniotic twins, and later splitting leads to conjoined twins.

Option by option

A. Within the first 3 days, at the morula stage
Splitting before day 4 would give two chorions and two placentas or fused placentas with lambda sign.
B. Between day 8 and day 12, at the bilaminar disc stage
Splitting at days 8 to 12 would give a single amniotic sac.
C. Between day 4 and day 8, at the blastocyst stage (correct)
Correct. Division of the inner cell mass at the blastocyst stage gives a single chorion and two amnions.
D. After day 13, at the primitive streak stage
Splitting after day 13 would give conjoined twins.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Anatomy question

What is the answer to this Anatomy question?

The best answer is C, Between day 4 and day 8, at the blastocyst stage. Splitting of the inner cell mass at days 4 to 8 gives monochorionic diamniotic twins, the most common monozygotic type.

Why is "Within the first 3 days, at the morula stage" not the answer?

Splitting before day 4 would give two chorions and two placentas or fused placentas with lambda sign.

Why is "Between day 8 and day 12, at the bilaminar disc stage" not the answer?

Splitting at days 8 to 12 would give a single amniotic sac.

Why is "After day 13, at the primitive streak stage" not the answer?

Splitting after day 13 would give conjoined twins.

What concept does this question test?

It tests Embryology: twinning in Anatomy. Splitting before day 4 produces dichorionic diamniotic twins, splitting at days 8 to 12 gives monochorionic monoamniotic twins, and later splitting leads to conjoined twins.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Embryology: twinning?

Learn every nerve by its route, its muscles and what a lesion looks like at the bedside. That one habit answers a large share of questions. Then use the flashcards and the other Anatomy questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.