**Core Concept**
Mass spectrometry is a highly sensitive analytical technique used to identify and quantify the chemical composition of a sample by measuring the mass-to-charge ratio of ions. This technique is widely used in various fields, including pharmaceuticals, biotechnology, and clinical diagnostics, to detect and identify small molecules, peptides, proteins, and other biomolecules.

**Why the Correct Answer is Right**
Mass spectrometry works by ionizing the sample into charged particles, which are then separated based on their mass-to-charge ratio using magnetic or electric fields. The resulting mass spectra provide a unique fingerprint of the sample's chemical composition, allowing for the identification and quantification of specific molecules. This technique is particularly useful for detecting small molecules, such as drugs, metabolites, and toxins, in biological fluids.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because mass spectrometry is not typically used for DNA sequencing. While mass spectrometry can be used to analyze the mass of nucleic acids, it is not a primary technique for DNA sequencing.

**Option B:**
This option is incorrect because while mass spectrometry can be used to analyze the composition of complex mixtures, it is not typically used for electron microscopy. Electron microscopy is a technique that uses a beam of electrons to produce an image of the sample's surface morphology.

**Option C:**
This option is incorrect because mass spectrometry is not typically used for X-ray crystallography. X-ray crystallography is a technique that uses X-rays to determine the atomic structure of a crystal.

**Clinical Pearl / High-Yield Fact**
Mass spectrometry is a highly sensitive and specific technique that can detect small molecules in complex biological samples, making it a valuable tool in clinical diagnostics and pharmaceutical research.

**Correct Answer: D. Mass spectrometry is used for identifying and quantifying the chemical composition of a sample.**