UHS MDCAT 2025 past paper, Paper D

Answer: C. Hypertonic urine with decreased volume

Antidiuretic Hormone (ADH) increases water reabsorption in the kidneys, leading to more concentrated (hypertonic) urine. This increased reabsorption reduces the overall volume of urine produced. Option D is tempting because ADH causes concentration, but it's incorrect as the volume decreases, not increases.

Answer: A. Two identical double helical DNA molecules

A duplicated chromosome consists of two identical sister chromatids, and each chromatid contains one double-helical DNA molecule. Therefore, a mature duplicated chromosome has two identical DNA molecules. Option B is incorrect because the DNA molecules in sister chromatids are identical copies. Option C is incorrect because the DNA is organized into distinct molecules within each chromatid, not distributed between them. Option D describes the structure of chromatin, not the entire duplicated chromosome.

Answer: C. Histone

Histones are proteins that DNA wraps around to form nucleosomes, which are the basic units of chromatin. This coiling and folding process is essential for packaging the long DNA molecules into compact chromosomes within the

Answer: A. Syphilis

The bacterium *Treponema pallidum* is the causative agent of syphilis. While other options are also sexually transmitted infections, they are caused by different pathogens: Chlamydia is caused by *Chlamydia trachomatis*, Gonorrhea by *Neisseria gonorrhoeae*, and Genital herpes by the Herpes Simplex Virus.

Answer: D. Stretching of necks over generations

Lamarck believed that organisms acquired traits through use and disuse, and then passed these acquired traits to their offspring. Therefore, giraffes stretching their necks to reach higher leaves would lead to longer necks, a trait inherited by subsequent generations. Option A, Natural selection, is Darwin's theory, which involves differential survival and reproduction based on inherited traits, not acquired ones.

Answer: B. 77.8%

The limiting reactant is O₂ because 1 mole of O₂ (32g) reacts with 2 moles of H₂ (4g) according to the stoichiometry. The theoretical yield of water is calculated using the limiting reactant: 1 mole of O₂ produces 2 moles of H₂O, which is 36g. The percentage yield is (28g actual yield / 36g theoretical yield) * 100% = 77.8%.

Answer: D. heat and UV light

Free radical substitution reactions, like the halogenation of ethane, require energy to initiate the formation of free radicals. This energy is typically provided by heat or ultraviolet (UV) light, which breaks the halogen molecule into highly reactive halogen radicals. Low temperatures (B) would inhibit radical formation, while pressure (A, C) is not the primary requirement for initiating this type of reaction.

Answer: D. Amide

When a carboxylic acid reacts with ammonia, it forms an ammonium salt. Upon heating, this salt undergoes dehydration, eliminating a molecule of water to form an amide. The most tempting wrong option is A) Carbonates, as heating some ammonium salts can lead to carbonates, but this is not the specific product when the starting material is a carboxylic acid-derived ammonium salt.

Answer: B. lower pressure on the top

The curved shape of an airplane wing causes air to travel a longer distance over the top surface than the bottom surface in the same amount of time. According to Bernoulli's principle, faster-moving air exerts lower pressure. Therefore, the air moving faster over the top creates lower pressure on the top surface compared to the bottom surface. Option A is incorrect because greater pressure on the top would push the wing down, not lift it up.

Answer: A. 4 m/s

The relative speed of separation after an elastic collision is equal to the relative speed of approach. Since the bodies are moving in the same direction, the relative speed of approach is the difference between their velocities (5 m/s - 1 m/s = 4 m/s). Therefore, the relative speed of separation will also be 4 m/s, assuming an elastic collision.