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USMLE® Step 1 style questions USMLE

1 questions

A laboratory physician investigates the chromosomes of a fetus with a suspected chromosomal anomaly. She processes a cell culture obtained by amniocentesis. The results of the analysis are shown below. Based on the karyotype analysis, which of the following is the most likely diagnosis?  

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Content Reviewers:

Viviana Popa, MD

Karyotyping is the simple process of seeing what a person’s chromosomes look like.

But don’t think of it as a chromosome beauty contest - karyotyping is actually used to detect chromosome number or structure abnormalities, in order to diagnose genetic disorders, like Down syndrome; or even some types of cancer, like leukemia.

Ok, now, chromosomes are found in the nucleus of our cells and they contain our DNA.

You can think of DNA like a library, with thousands of books called genes that carry recipes for how to make every single protein found in the cell.

In human somatic cells, so all cells besides the gametes; there are 23 pairs of homologous chromosomes, and in each pair, one chromosome came from each parent.

This adds up to 46 chromosomes in total.

Of these 23 pairs, 22 are somatic pairs, which contain genes that code for traits like hair color; and one sexual pair, which determines the biological sex of an individual.

In genetically female individuals, there are two X chromosomes, while in genetically male individuals, there’s an X and a Y chromosome.

However, chromosomes actually look different depending on the phases of the cell cycle - which is the series of events that somatic cells go through from the moment they’re formed until they divide into two identical daughter cells.

The cell cycle has two phases: interphase, when the cell prepares to divide, and mitosis, when the cell is dividing.

In early interphase, each chromosome has a single copy of the genetic information, called a chromatid, so there’s 46 chromosomes and 46 chromatids.

But in later interphase, as the cell prepares for mitosis, each chromosome is copied and pasted, so the amount of DNA, aka the number of chromatids, doubles up.

But two identical chromatids remain joined in a region called the centromere - so they still count as one chromosome.

So right before mitosis, there are 46 chromosomes and 92 chromatids.

This way, the two resulting daughter cells have 46 chromosomes and 46 chromatids each, so they have the same DNA as the original cell.

Ok, now karyotyping is actually done by snapping a picture of the chromosomes during mitosis - because that’s when they are at their most condensed, and they’re the most visible.

Mitosis can be broken down into prophase, metaphase, anaphase, and telophase.

And metaphase is when the chromosomes, made up of two chromatids each, neatly align on the midline of the cell, like 46 little X shapes - where each side of the X is a chromatid.

So, in order to select chromosomes in metaphase, first you need to choose cells that can easily enter mitosis.

And a good option are white blood cells from a blood sample.

Next, white blood cells are incubated in a culture flask with growth factors, which stimulate mitosis.

After a while, when a number of cell divisions have taken place, a substance called colchicine is added, which stops mitosis from progressing past metaphase.

Then, the cells are transferred to a conical tube, and a hypotonic solution is added.

Hypotonic means it’s more dilute than the intracellular fluid.

The laws of osmosis dictate that fluid goes from the more dilute compartment to the more concentrated compartment - so the cells swell with fluid and eventually burst like a balloon, releasing the chromosomes in the solution.

Then, a drop of the solution is put in a glass slide and preserved with a chemical fixative, like Carnoy’s fluid, which is a mixture of acetic acid and ethanol.

Next, the chromosomes are stained, in order to identify the chromosome sections that are lightly packed and carry mostly genes, called euchromatin; and the parts that are more condensed and don’t have as many genes, called heterochromatin.

These are seen as alternating bright and dark bands that appear along each chromosome, called banding patterns.

These are different depending on the technique and stain used.

The most common is G-banding, where the chromosomes are stained with Giemsa.

This stain attaches to regions that are rich in adenine and thymine pairs, like heterochromatin, making them darker.

In contrast, Giemsa doesn’t attach as much to euchromatin, which has more guanine and cytosine pairs.

So these regions are the bright bands.

Now, there’s other techniques that are used for different purposes. R-banding, for example, also uses Giemsa, but with a twist, so it shows a Reverse pattern of G-banding.

So in this case, euchromatin is dark, while heterochromatin is bright.

C-banding, is a technique that uses Giemsa to stain and study mostly the Centromeres.

Finally, Q-banding uses a fluorescent stain called Quinacrine, which creates a similar pattern as G-banding but can only be seen under UV light.

Now, once the chromosomes are stained, they’re easy to sort based on their size and the position of their centromere.


Karyotyping is the process of examining chromosomes to identify structural changes. It is used to detect genetic disorders and can also be used to determine the biological sex of an individual. The results of a karyotype can help to diagnose chromosomal abnormalities such as an abnormal number of chromosomes, called aneuploidy; or a structural abnormality, such as deletions, duplications, inversions, or translocations. Examples of genetic conditions diagnosed through karyotyping include Down syndrome, Turner syndrome, and Klinefelter syndrome.