UHS MDCAT 2023 past paper, Paper D

Answer: A. Vestigial organs

Vestigial organs are reduced or non-functional structures that were functional in ancestral species, indicating evolutionary history. Homologous organs share a common ancestor but may have different functions, while analogous organs have similar functions but different evolutionary origins. Embryonic organs refer to structures developing during the embryonic stage, not necessarily linked to ancestral functionality in the same way vestigial organs are.

Answer: C. Birds

Birds and mammals have a four-chambered heart, but only birds retain a single systemic arch, the right aortic arch. This arrangement, along with the four-chambered heart, is crucial for efficiently delivering oxygenated blood to support their high metabolic rate, especially during flight. While reptiles also have a three-chambered heart, their circulatory system differs significantly, and amphibians have an even simpler, less efficient system.

Answer: A. Interferon

Interferons are signaling proteins produced by host cells in response to viral infection. They interfere with viral replication by signaling neighboring cells to activate antiviral defenses. Histones are proteins involved in DNA packaging, antigens trigger an immune response but don't directly control viruses, and serum contains antibodies but interferons are the direct molecular controllers released by infected cells.

Answer: A. AgNO₃ + NH₄OH

Option A is correct because terminal alkynes like 1-pentyne react with ammoniacal silver nitrate to form a white precipitate (silver acetylide). 2-Pentyne, being an internal alkyne, lacks the acidic hydrogen and thus does not react. Option B, alkaline KMnO₄, would react with both alkynes through oxidation, making it unsuitable for distinction. Option C, Br₂/CCl₄, would cause decolorization with both alkynes due to addition reactions. Option D, K₂Cr₂O₇ +H₂SO₄, is a strong oxidizing agent and would likely lead to complex reactions with both, not a clear distinction.

Answer: D. Addition

Alkanes are saturated hydrocarbons, meaning they only contain single bonds. Addition reactions involve breaking double or triple bonds to add atoms, which alkanes lack. Substitution, combustion, and hydrogenation are all reactions that alkanes can undergo due to the relative stability of their C-C and C-H single bonds.

Answer: A. CH₃COCl + SO₂ + HCl

This reaction is a standard method for converting carboxylic acids into acyl chlorides. Acetic acid (CH₃COOH) reacts with thionyl chloride (SOCl₂) to produce acetyl chloride (CH₃COCl), sulfur dioxide (SO₂), and hydrogen chloride (HCl). Option A accurately lists all these products. Option D is tempting because it includes HCl and SO₂, but it incorrectly suggests CH₃Cl as a product instead of CH₃COCl.

Answer: B. 4W

The stored energy in a capacitor is given by the formula $W = Q^2 / (2C)$. If the charge is increased to $2Q$, the new energy $W'$ becomes $(2Q)^2 / (2C) = 4Q^2 / (2C) = 4W$. Option A is tempting because it suggests a linear relationship, but the energy depends on the square of the charge.

Answer: C. Perpendicular to magnetic field

The force on a current-carrying conductor in a magnetic field is given by the formula $F = BIL\sin\theta$, where $\theta$ is the angle between the conductor and the magnetic field. This force is maximum when $\sin\theta$ is maximum, which occurs when $\theta = 90^\circ$, meaning the conductor is perpendicular to the magnetic field. Option D, antiparallel, would result in zero force because $\sin(180^\circ) = 0$.

Answer: D. 100

The gray (Gy) and rad are both units of absorbed radiation dose. The gray is the SI unit, and 1 gray is defined as 1 joule of absorbed energy per kilogram of mass. The rad is an older unit, and the conversion factor is 1 Gy = 100 rad. Option D is correct because it accurately reflects this conversion. Option A is incorrect as it represents 1 rad = 0.01 Gy.

Answer: A. Trapped by the spider

The most appropriate word is "trapped" because the common understanding of a fly's interaction with a spider is that it becomes prey. While a spider might "entertain" or "invite" a fly in a metaphorical sense, the typical outcome is capture and consumption. "Spared" is unlikely given the predatory nature of spiders.