Which of the following enzymes requires acidic medium to function?

Analyze the question: We need the enzyme that works best in an acidic medium (low pH).

Concept: Enzymes have an optimum pH.

• Mouth (saliva): slightly alkaline/neutral, so salivary amylase (ptyalin) works here.
• Stomach: strongly acidic (pH about 1.5-2) because of hydrochloric acid from the gastric glands, so enzymes here work in acid.
• Small intestine: alkaline (bile, pancreatic juice, and intestinal juice), so pancreatic enzymes and erepsin work here.

Checking each option:

• A. Amylase: Breaks down starch to maltose. It works in neutral or slightly alkaline medium (salivary and pancreatic amylase). Not the answer.
• B. Erepsin: Found in the intestinal juice, it converts peptides to amino acids in an alkaline medium. Not the answer.
• C. Maltose: This is a sugar, not an enzyme (the enzyme would be maltase). Not the answer.
• D. Pepsin: Secreted in the stomach as inactive pepsinogen, it is activated by HCl and converts proteins to peptides at pH of about 2. It requires an acidic medium. Correct.
• E. Ptyalin: Another name for salivary amylase, which works in a slightly alkaline or neutral medium in the mouth. Not the answer.

Answer: Pepsin, option D.

Tip: Link "Pepsin" with "Protein" and "Pot of acid" (stomach). Remember that the stomach is acidic, and almost everything else in digestion (mouth, small intestine) is neutral to alkaline.