Embryology and Development Practice Questions

Q: Atavism is the resemblance of characteristics to which ancestral generation?

Grandparents. **Explanation:** **Atavism** (also known as evolutionary throwback) refers to the reappearance of a biological trait in an organism after several generations of absence. In the context of medical genetics and embryology, it is defined as the resemblance of characteristics to **grandparents** or even more remote ancestors, rather than the immediate parents. 1. **Why Grandparents is correct:** Atavism occurs when genes for previously expressed ancestral traits—which were preserved in the DNA but "silenced" or suppressed during evolution—are suddenly reactivated due to genetic recombination or mutations. Since these traits skip the immediate generation (parents), the closest ancestral resemblance is noted in the grandparents or beyond. 2. **Why other options are wrong:** * **Parents:** Resemblance to parents is termed "heredity" or direct inheritance. * **Siblings:** Resemblance among siblings is due to shared parental alleles and is not considered an evolutionary throwback. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Examples of Atavism in Humans:** * **Atavistic Tail:** The presence of a true vestigial tail (containing muscle and blood vessels) in a newborn. * **Polythelia:** Presence of accessory nipples along the milk line. * **Persistent Lanugo:** Excessive body hair resembling an ancestral coat. * **Distinction:** Do not confuse Atavism with **Vestigial organs**. Vestigial organs (like the vermiform appendix) are present in all members of a species but have lost their original function, whereas Atavistic traits appear only in specific individuals. * **Mechanism:** It usually results from the loss of "suppressor genes" that normally keep ancestral developmental pathways dormant.

Q: What is the most common site of fertilization?

Ampulla. **Explanation:** Fertilization is the process where the male and female gametes fuse to form a zygote. In humans, this occurs within the **Fallopian tube (uterine tube)** [1]. **Why Ampulla is the Correct Answer:** The **Ampulla** is the widest and longest part of the fallopian tube (approximately 5 cm long). Due to its spacious lumen and its location distal to the isthmus, it serves as the most favorable environment for the sperm to meet the oocyte. Statistically, it is the site of fertilization in over 90% of cases [1]. **Analysis of Incorrect Options:** * **Isthmus (A):** This is the narrow, thick-walled medial part of the tube. While sperm pass through it, its narrow lumen is not the primary site for fertilization. * **Infundibulum (C):** This is the funnel-shaped lateral end with fimbriae that "catch" the ovum from the ovary [1]. It is the entry point for the egg, not the site of fusion. * **Interstitial/Intramural (D):** This is the segment that pierces the uterine wall. It is the narrowest part of the tube and is not involved in fertilization. **NEET-PG High-Yield Pearls:** 1. **Ectopic Pregnancy:** The **Ampulla** is also the most common site for an ectopic pregnancy. 2. **Narrowest Part:** The **Interstitial (Intramural) part** is the narrowest portion of the fallopian tube (0.7 mm). 3. **Fertilization Timing:** Fertilization typically occurs within **12–24 hours** after ovulation. 4. **Implantation:** While fertilization occurs in the ampulla, implantation usually occurs in the **posterior wall of the body of the uterus** on approximately the 6th day after fertilization [1].

Q: How many vessels are typically seen in a cross-section of the umbilical cord?

Three. **Explanation:** The umbilical cord typically contains **three vessels**: **two umbilical arteries** and **one umbilical vein** [1], all embedded in a gelatinous substance called **Wharton’s jelly**. 1. **Why Three is Correct:** During early development, there are initially four vessels (two arteries and two veins). However, the **right umbilical vein** undergoes regression (disappears) around the 8th week of gestation, leaving only the **left umbilical vein** and the two umbilical arteries. The vein carries oxygenated blood from the placenta to the fetus, while the arteries carry deoxygenated blood from the fetus back to the placenta [1]. 2. **Why Other Options are Incorrect:** * **One:** A single vessel is never normal and is incompatible with life. * **Two:** A **Single Umbilical Artery (SUA)** occurs in about 1% of pregnancies. While it can be an isolated finding, it is highly associated with congenital anomalies (renal or cardiac) and chromosomal trisomies, making it a significant clinical finding rather than the "typical

Q: The upper three-fourths of the vagina develops from which embryonic structure?

Mullerian duct. **Explanation:** The development of the female reproductive tract is a high-yield topic in embryology. The vagina has a **dual origin**, arising from two distinct embryonic structures: 1. **Mullerian (Paramesonephric) Ducts:** The caudal ends of the Mullerian ducts fuse to form the **uterovaginal canal**. This canal gives rise to the uterus, cervix, and the **upper 3/4th (upper portion)** of the vagina [1]. 2. **Urogenital Sinus (Sinovaginal Bulbs):** The lower part of the uterovaginal canal meets the urogenital sinus, inducing the formation of the **sinovaginal bulbs**. These bulbs proliferate and canalize to form the **lower 1/4th** (or 1/5th) of the vagina [1]. **Analysis of Options:** * **Option A (Correct):** As stated, the Mullerian ducts form the upper 3/4th of the vagina, along with the fallopian tubes and uterus [1]. * **Option B (Incorrect):** The **Wolffian (Mesonephric) ducts** regress in females due to the absence of testosterone. Remnants may persist as **Gartner’s cysts** in the lateral wall of the vagina. * **Option C (Incorrect):** The sinovaginal bulbs form the **lower 1/4th** of the vagina, not the upper portion [1]. * **Option D (Incorrect):** While the urogenital sinus is endodermal, the upper 3/4th of the vagina is derived from the Mullerian duct, which is **mesodermal** in origin [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Mayer-Rokitansky-Küster-Hauser (MRKH) Syndrome:** Congenital absence of the uterus and the upper 2/3rd to 3/4th of the vagina due to Mullerian agenesis. * **Hymen:** Formed at the junction where the sinovaginal bulbs meet the urogenital sinus. * **Epithelium:** Initially, the entire vagina is lined by columnar epithelium (Mullerian), which is later replaced by **stratified squamous epithelium** from the urogenital sinus.

Q: All of the following are components of the placental barrier, EXCEPT:

Amnion. The **placental barrier** (placental membrane) is a composite structure that separates the maternal blood in the intervillous spaces from the fetal blood within the villi [1]. Its primary function is to prevent the mixing of blood while allowing the exchange of nutrients, gases, and waste [3]. ### Why Amnion is the Correct Answer The **Amnion** is the innermost fetal membrane that encloses the amniotic cavity and the fetus [1]. While it is part of the fetal membranes, it **does not contribute to the structure of the chorionic villi** where gas and nutrient exchange occurs [1]. Therefore, it is not a component of the placental barrier. ### Explanation of Other Options The placental barrier is composed of four layers (from external to internal): * **Trophoblast (Option A):** Specifically the outer **Syncytiotrophoblast** (which is in direct contact with maternal blood) and the inner **Cytotrophoblast** [5]. * **Mesoderm (Option C):** The extraembryonic mesoderm forms the connective tissue core of the villus, providing structural support [4]. * **Fetal capillary endothelium (Option B):** The innermost layer that lines the fetal blood vessels within the villi [4]. ### High-Yield Facts for NEET-PG * **Thinning of the Barrier:** In early pregnancy, all four layers are present. In late pregnancy (after the 4th month), the barrier thins to facilitate faster exchange; the **cytotrophoblast** and **mesoderm** largely disappear, leaving only the syncytiotrophoblast and fetal endothelium. * **The "Barrier" is not absolute:** Most drugs with a molecular weight <600 Da, viruses (e.g., Rubella, CMV, HIV), and certain antibodies (IgG) can cross this barrier [3]. * **IgG Transfer:** IgG is the only immunoglobulin that crosses the placenta, providing passive immunity to the newborn [2].

Q: The stapes is a derivative of which embryonic pharyngeal arch?

Second arch. The pharyngeal (branchial) arches are fundamental to head and neck development. Each arch contains a central cartilaginous rod that serves as a precursor to specific skeletal structures [1]. **Correct Answer: B. Second arch (Reichert’s cartilage)** The second pharyngeal arch is responsible for forming the **stapes** (except for its vestibular part/footplate, which is partly derived from the neural crest and otic capsule). Other key skeletal derivatives of the second arch include the **styloid process** of the temporal bone, the **stylohyoid ligament**, and the **lesser cornu and upper part of the body of the hyoid bone**. **Explanation of Incorrect Options:** * **A. First arch (Meckel’s cartilage):** This arch gives rise to the **malleus** and **incus** (the other two middle ear ossicles), as well as the mandible, maxilla, and zygomatic bone. * **C. Third arch:** This arch forms the **greater cornu and lower part of the body of the hyoid bone**. * **D. Fourth arch:** This arch (along with the sixth) contributes to the **laryngeal cartilages** (thyroid, cricoid, arytenoid, etc.), excluding the epiglottis. **High-Yield Clinical Pearls for NEET-PG:** * **Nerve Supply:** Remember the "Rule of Nerves": 1st Arch = Trigeminal (V); 2nd Arch = **Facial (VII)**; 3rd Arch = Glossopharyngeal (IX); 4th/6th Arch = Vagus (X). Since the stapes is a 2nd arch derivative, it is moved by the **stapedius muscle**, which is supplied by the **Facial nerve** [1]. * **Treacher Collins Syndrome:** Results from the failure of neural crest cell migration into the **first and second arches**, leading to malformation of the ossicles (conductive hearing loss) and zygomatic hypoplasia. * **Mnemonic:** The **S**tructures of the **S**econd arch all start with **S**: **S**tapes, **S**tyloid process, **S**tylohyoid ligament, and **S**even (CN VII).

Q: Which of the following placental anomalies is characterized by an abnormal attachment of the umbilical cord?

Battledore placenta (Marginal insertion). ### Explanation **Correct Answer: B. Battledore placenta (Marginal insertion)** The umbilical cord normally inserts into the center of the placental mass (eccentric or central insertion) [3]. In a **Battledore placenta**, the cord is attached at or within 2 cm of the **placental margin** [1]. The name is derived from its resemblance to a "battledore" (a historical badminton racket). While often clinically insignificant, it can be associated with preterm labor, fetal distress, or progress to a velamentous insertion. **Analysis of Incorrect Options:** * **A. Succenturiate lobe:** This is an anomaly of **placental morphology**, not cord attachment [2]. It involves one or more small accessory lobes of placental tissue situated at a distance from the main placental mass, connected by fetal vessels running through the membranes [2]. * **C. Horseshoe placenta:** This is a rare morphological variant where the placenta is shaped like a horseshoe, often associated with placenta previa. It does not specifically define an anomaly of cord attachment. **High-Yield Clinical Pearls for NEET-PG:** 1. **Velamentous Insertion:** A more severe attachment anomaly where the cord inserts into the **fetal membranes** (chorioamniotic membranes) rather than the placental mass itself [1]. The vessels run unprotected by Wharton’s jelly. 2. **Vasa Previa:** Often associated with velamentous insertion or succenturiate lobes; fetal vessels cross the internal os [1]. This is a surgical emergency due to the risk of fetal exsanguination upon rupture of membranes. 3. **Placenta Membranacea:** A rare condition where the entire chorionic sac is covered by functional villi, resulting in a thin, membranous placenta [2].

Q: A baby girl presents with bilateral inguinal masses, thought to be hernias but are found to be testes in the inguinal canals. Which karyotype would you expect to find in the child?

46,XY. ### Explanation **Correct Answer: B. 46,XY** **Concept:** The clinical presentation describes a classic case of **Androgen Insensitivity Syndrome (AIS)**, formerly known as Testicular Feminization Syndrome. In AIS, there is a mutation in the androgen receptor gene. Although the individual has a **46,XY karyotype** and functional testes that produce testosterone and Anti-Müllerian Hormone (AMH), the peripheral tissues cannot respond to androgens [1]. 1. **Testicular Development:** The presence of the SRY gene on the Y chromosome ensures the development of testes. 2. **Phenotype:** Because the body cannot respond to testosterone/DHT, external genitalia develop along female lines (default pathway). 3. **Clinical Clue:** The "inguinal hernias" in a phenotypic female infant are actually **undescended testes** located in the inguinal canal or labia majora. AMH causes regression of the uterus and fallopian tubes, leading to a blind-ending vaginal pouch. --- **Analysis of Incorrect Options:** * **A. 46,XX:** This is a normal female karyotype. While inguinal hernias can occur in females (containing ovaries), they would not contain histologically confirmed testes. * **C & D. 46,XXY / 47,XXY (Klinefelter Syndrome):** These individuals have a male phenotype (small testes, gynecomastia, tall stature) because they respond to androgens. They do not present as phenotypic females with inguinal masses in infancy. --- **High-Yield NEET-PG Pearls:** * **Most common cause of primary amenorrhea with absent uterus:** AIS (46,XY) or Müllerian Agenesis (46,XX). * **AIS vs. Müllerian Agenesis:** In AIS, testosterone levels are in the normal male range and pubic/axillary hair is absent or sparse. * **Management:** Testes are usually left in situ until after puberty to allow for natural estrogenization (via peripheral aromatization of testosterone), then removed to prevent **Gonadoblastoma/Dysgerminoma**.

Question 1

Atavism is the resemblance of characteristics to which ancestral generation?

Accepted Answer

Grandparents

Answer

Parents

Answer

Siblings

Answer

None of the above

Question 2

What is the most common site of fertilization?

Accepted Answer

Ampulla

Answer

Isthmus

Answer

Infundibulum

Answer

Interstitial

Question 3

How many vessels are typically seen in a cross-section of the umbilical cord?

Accepted Answer

Three

Answer

One

Answer

Four

Answer

Two

Question 4

In renal agenesis, where is the adrenal gland typically located?

Accepted Answer

Present at the usual location

Answer

Absent

Answer

Present on the contralateral side

Answer

Ectopic in the iliac fossa

Question 5

The upper three-fourths of the vagina develops from which embryonic structure?

Accepted Answer

Mullerian duct

Answer

Wolffian duct

Answer

Sinovaginal bulb

Answer

Endoderm

Question 6

All of the following are components of the placental barrier, EXCEPT:

Accepted Answer

Amnion

Answer

Trophoblast

Answer

Fetal capillary endothelium

Answer

Mesoderm

Question 7

The stapes is a derivative of which embryonic pharyngeal arch?

Accepted Answer

Second arch

Answer

First arch

Answer

Third arch

Answer

Fourth arch

Question 8

What is true about Lyonization of the X chromosome?

Accepted Answer

Inactivation of the X chromosome occurs only in somatic cells.

Answer

Inactivation of the X chromosome occurs only in germ cells.

Answer

Inactivation of the X chromosome occurs in both somatic and germ cells.

Answer

The maximum number of Barr bodies is equal to the number of X chromosomes.

Question 9

Which of the following placental anomalies is characterized by an abnormal attachment of the umbilical cord?

Accepted Answer

Battledore placenta (Marginal insertion)

Answer

Succenturiate lobe (Lobate placenta)

Answer

Horseshoe placenta

Answer

None of the above

Question 10

A baby girl presents with bilateral inguinal masses, thought to be hernias but are found to be testes in the inguinal canals. Which karyotype would you expect to find in the child?

Accepted Answer

46,XY

Answer

46,XX

Answer

46,XXY

Answer

47,XXY

Embryology and Development — MCQs

Embryology and Development — MCQs

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