**Question:** Diatoms are seen in

A. Blood vessels
B. Skin lesions
C. Urine
D. Sputum

**Core Concept:** Diatoms are a type of microorganism, specifically a group of algae, that contain silica shells called frustules. These shells can be observed in various biological samples during microscopic examination.

**Why the Correct Answer is Right:** Diatoms are indeed found in the sputum of patients with pulmonary infections, where they can serve as opportunistic pathogens. Their presence in sputum can indicate an infection or contamination, depending on the clinical context.

**Why Each Wrong Option is Incorrect:**

A. Diatoms are not typically found in the blood vessels. Their presence might indicate contamination or a rare case of disseminated infection.
B. Diatoms are not typically found in skin lesions. They are not involved in skin diseases and their presence could be due to contamination or an unusual infection.
C. Diatoms are not commonly found in urine. In rare cases, they might be present due to contamination or a rare case of disseminated infection.

**Clinical Pearl:** To differentiate between contamination and infection, consider the clinical context, patient's history, and other associated findings during examination.

**Correct Answer:** D. Sputum

Diatoms are often found in sputum samples, particularly in patients with pulmonary infections. Upon microscopic examination, their silica shells can be observed in the context of a patient's clinical presentation and other findings, helping to determine the presence of infection or contamination.