**Question:** All of the following drugs cause discolouration of urine EXCEPT -

A. Phenazopyridine
B. Indomethacin
C. Prazosin
D. Paracetamol

**Core Concept:**
Drugs that can cause discolouration of urine can be classified into two main categories: those that directly alter the chemical composition of urine or those that have a non-specific effect on urine production and composition. Discolouration can be due to the presence of certain molecules or compounds that affect the wavelength of light absorbed or reflected by urine, thereby changing its colour.

**Why the Correct Answer is Right:**
Drugs causing discolouration of urine can be categorized into two groups: those that directly alter urine composition (e.g., containing pigments or products of drug biotransformation) or those that affect urine production (e.g., diuretics) and urine composition (e.g., increasing protein or blood content).

**Why Each Wrong Option is Incorrect:**

A. Phenazopyridine: It is a urinary antispasmodic and analgesic that is used for the relief of pain and frequency associated with urinary tract infections. It directly alters urine composition by increasing the concentration of urochrome, a yellow-coloured compound, leading to yellow or brown urine.

B. Indomethacin: It is a non-steroidal anti-inflammatory drug (NSAID) that inhibits the production of prostaglandins. Prostaglandins play a crucial role in maintaining the physiological balance of urine colour. Indomethacin does not alter urine composition directly but indirectly affects urine production, leading to a decrease in urine volume and a higher concentration of solutes, causing discolouration.

C. Prazosin: It is a selective alpha-1 blocker used for the treatment of hypertension and benign prostatic hyperplasia. It does not directly alter urine composition but affects urine production, leading to a decrease in urine volume and a higher concentration of solutes, causing discolouration.

D. Paracetamol: It is a commonly used analgesic and antipyretic medication. It does not directly alter urine composition or affect urine production, but does not cause discolouration in most cases, indicating that it is not a strong candidate for altering urine colour.

**Core Concept:** Drugs can directly alter urine colour by increasing the concentration of pigmented compounds or indirectly affect urine production, leading to a decrease in urine volume and an increase in solute concentration, causing discolouration.

**Why Each Wrong Option is Incorrect:**

A. Phenazopyridine: This drug increases the concentration of urochrome, a yellow pigmented compound, leading to discolouration of urine.

B. Indomethacin: Although indomethacin is an NSAID, it primarily affects urine production and does not directly alter urine composition, making it less likely to cause discolouration compared to drugs like phenazopyridine or prazosin.

C. Prazosin: Similar to indomethacin, prazosin does not directly alter urine composition but affects