In the human body, the 'Blood-Brain Barrier' (BBB) is highly selective. Which of the following substances can easily cross the BBB? MCQ with Answer and Explanation

Question

In the human body, the 'Blood-Brain Barrier' (BBB) is highly selective. Which of the following substances can easily cross the BBB?

A. Bacteria and viruses

B. Glucose, oxygen, and carbon dioxide

C. Most water-soluble drugs

D. Large proteins and peptides

Answer: Option B

Solution (By JKExamLibrary)

The Blood-Brain Barrier (BBB) is formed by tight junctions between endothelial cells in brain capillaries, making it highly selective. It allows the free diffusion of small, lipid-soluble molecules and essential gases. Therefore, oxygen, carbon dioxide, and lipid-soluble substances (like alcohol, nicotine, and some anesthetics) easily cross. Glucose, a water-soluble molecule essential for brain energy, crosses via specific carrier-mediated transport (GLUT1 transporters). Large proteins, peptides, most water-soluble drugs, and pathogens like bacteria and viruses are generally blocked from entering the brain parenchyma.

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Question #1

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The deficiency of iodine in the diet leads to:

A. Anemia

B. Rickets

C. Scurvy

D. Goiter

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Correct Answer: Option D

Explanation:
Iodine is essential for synthesis of thyroxine (T₄) and triiodothyronine (T₃). Deficiency leads to enlargement of the thyroid gland, known as goiter, as the gland tries to compensate. Anemia is due to iron deficiency, rickets to vitamin D deficiency, scurvy to vitamin C deficiency. Iodized salt is a public health measure to prevent goiter.

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Question #2

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In molecular biology, the 'Wobble hypothesis' explains the flexibility in base pairing between the:

A. First base of the codon and the first base of the anticodon

B. Third base of the codon and the first base of the anticodon

C. DNA template and the newly synthesized mRNA

D. Second base of the codon and the second base of the anticodon

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Correct Answer: Option B

Explanation:
The Wobble hypothesis, proposed by Francis Crick, explains the degeneracy of the genetic code. It states that the base pairing rules (A-U, G-C) are strictly followed for the first two bases of the mRNA codon, but the third base of the codon (the 3' end) can form non-standard or 'wobble' base pairs with the first base of the tRNA anticodon (the 5' end). This allows a single tRNA molecule to recognize and bind to more than one synonymous codon.

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Question #3

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Phenylketonuria (PKU) is an example of:

A. Polygenic inheritance

B. Epistasis

C. Pleiotropy

D. Sex-linked inheritance

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Correct Answer: Option C

Explanation:
Pleiotropy is a phenomenon where a single gene influences multiple, seemingly unrelated phenotypic traits. Phenylketonuria (PKU) is an autosomal recessive metabolic disorder caused by a mutation in a single gene (PAH) that codes for the enzyme phenylalanine hydroxylase. This single mutation leads to multiple effects: intellectual disability, reduced hair and skin pigmentation, and musty odor.

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