SIBA MDCAT 2025 past paper, Version C

Answer: A. Leafy vegetables

Leafy vegetables like spinach and rhubarb are high in oxalates, which can bind with calcium in the body to form calcium oxalate stones. While other options like fruits rich in Vitamin C or fiber, and whole grains, are generally healthy, they don't pose the same risk for calcium oxalate stone formation as high-oxalate foods.

Answer: D. Point of action is allosteric site

A non-competitive inhibitor binds to a site on the enzyme different from the active site, known as an allosteric site. This binding changes the enzyme's shape, reducing its ability to catalyze the reaction, regardless of substrate concentration. Option A is incorrect because competitive inhibitors bind to the active site. Option B is incorrect because non-competitive inhibition cannot be overcome by increasing substrate concentration. Option C is incorrect as inhibitors slow down reactions.

Answer: A. Female gamete

The defective gene is on the X chromosome. Females have two X chromosomes (XX), so they can carry the defective gene and pass it to their sons via their egg (female gamete). Males have one X and one Y chromosome (XY); they pass their Y chromosome to their sons, so they cannot transmit an X-linked defective gene to their male offspring.

Answer: A. Unicellular prokaryote

The organism is unicellular because it is made of a single cell. It lacks a nucleus, which is a defining characteristic of prokaryotes. Therefore, it is best classified as a unicellular prokaryote. Option B is tempting because it is unicellular, but it incorrectly suggests the presence of a nucleus.

Answer: B. Dendrite ..... cell body ...... axon

The correct answer is B because nerve impulses, or action potentials, are initiated in the dendrites, travel to the cell body, and are then transmitted away from the cell body along the axon. Option A is incorrect because impulses do not travel from the axon to the dendrite.

Answer: D. 73%

The percent ionic character can be calculated using

Answer: C. Zn/Hg conc.HCl

Clemmensen reduction specifically uses zinc amalgam (Zn/Hg) in concentrated hydrochloric acid (HCl) to reduce carbonyl groups of aldehydes and ketones to alkanes. Lithium aluminum hydride (LiAlH₄) is a strong reducing agent but typically reduces carbonyls to alcohols, not alkanes. Similarly, the Wolff-Kishner reduction (option D) also converts carbonyls to alkanes but uses a different set of reagents. Hydrogenation with H₂ and Pd (option A) usually reduces carbonyls to alcohols or alkenes, depending on the substrate and conditions.

Answer: B. HCHO

The carbonyl carbon in formaldehyde (HCHO) is the most electrophilic because it is bonded to two hydrogen atoms, which are weakly electron-donating. This results in a greater partial positive charge on the carbonyl carbon compared to other options. Acetic acid (CH₃COOH) is less electrophilic due to the electron-donating effect of the methyl group and the resonance stabilization involving the -OH group.

Answer: B. 4J

The work done by a gas at constant pressure is calculated by the formula W = PΔV, where P is the constant pressure and ΔV is the change in volume. In this case, P = 2 Pa and ΔV = 3 m³ - 1 m³ = 2 m³. Therefore, W = (2 Pa)(2 m³) = 4 J. Option A is incorrect because it doesn't account for the full change in volume.

Answer: A. The budget will be approved by the board only after the accounts are verified by the auditor.

Option A correctly transforms the active voice to passive voice while maintaining the original tense and meaning. The future tense "will approve" becomes "will be approved" and "verifies" becomes "are verified," accurately reflecting the conditional and future nature of the original sentence. Option B incorrectly changes the tense from future to present ("is approved").